1. SCREAMERS
   If you do have a screamer(and he's not screaming because he's hurt, sick, or neglected), try the "time out" method I use with my bird, and be sure to praise him like crazy after he's been quiet; you could also offer him a treat when after he's allowed back out and is being quiet.
   The first step to stopping the screaming is to determine WHY the bird is screaming in the first place. Some things to check are:
   
   1. Does the bird have ample fresh food and water?
   2. Is the bird shoved off in a back room or bedroom where it's alone for much of the day? Birds who feel isolated will scream in a desperate attempt to get some attention.
   3. Does your bird have a good number of safe, varied toys? Bored birds will scream just like a bored 2 year old will scream.
   4. Does your bird get enough exercise? Sometimes pent up energy gets re-routed through screaming.
   5. Is your bird in a tiny cage? See letter D.
   If your bird is screaming because it's isolated from the family and you absolutely can't, for whatever reason, relocate the cage to a more active part of the house, you can try responding to the bird in a quiet voice, with quiet whistled tunes, etc..this encourages the bird to make softer contact calls and sounds; in cases like this the bird simply wants to know that it hasn't been abandoned by the flock or that something hasn't happened to the flock. If you are in the room and the bird starts screaming, this can work as well. Whispering to the bird is also effective.
   
   Some things to ask youself if your bird has "suddenly" become a screamer:
   ...when you first got him:
   
   1. Did you spend every second of your day with him?
   2. Did you spoil him?
   3. Did you come running every time he'd call?
   4. Did you give him treats or take him out when he was noisy to "quiet him"?
   5. Did your daily schedule suddenly change? Example: You were previouslly unemployed/a student on vacation and home most of the day, and you've found work or have started classes again and are gone most of the day.
   
   If you did #3 or #4, you've successfully taught him to scream for attention. Good luck breaking that habit, it'll take a LOT of tolerance to a LOT of noise...tolerance from neighbors too. It can be done, but it takes the patience of Job. ;)
   
   If you did the first two, he's pretty much got every right to scream. He doesn't understand why you don't always spend time with him, and doesn't understand that your life may have changed and you CAN'T spend the time with him that you used to.
   He probably also doesn't know how to keep himself entertained if he's always had you for entertainment.
   All he knows is that for his life up to this point, he was the center of attention, and now he's not, and that's upsetting and confusing for him.
   
   Imagine how a human toddler would act if you doted on him for the first, say, year and a half of his life, then suddenly just started "ignoring" him or spending drastically less time with him...that kid would scream its' head off for some attention, and would likely NOT understand why attention levels suddenly dropped.
   That may be what your bird is doing.
   Unlike a human child, who will eventually understand the situation on his own, a bird usually will not.
   
   Un-spoiling a spoiled bird is a pretty big task.
   
   Curing a spoiled bird will also take a lot of tolerance to screams and yelling.
   
   Be sure he has a wide variety of toys that are different colors, sizes, and textures...try some of those toys that you can hide treats in him to try to get his interest.
   Do NOT give him attention while he's screaming, even yelling at him to be quiet is attention, and will only strengthen the bad habit.
   If you notice he's being quiet, go into the room(for now, drop whatever you're doing:)) and praise him for being a good boy. ONLY do this when he's being quiet, otherwise he'll probably still associate "calling" with "his flock returning"(hint: You're his flock:)).
   
   You can call from the other room with something like "I'm right here!" or "Hi!". I do call to my birds from the other room when they're calling to me to ask where I am..which may be what your bird is doing.
   Once they hear my voice and know that I'm still "alive", they usually calm down.
   
   Whatever you do, however, don't respond to screams with treats or by taking him out of his cage. If you let him out or give him a treat to quiet him down, you're teaching him that whenever he wants to come out or have a treat, all he has to do is scream. I made this mistake early on(he'd scream, I'd let him out/give him a treat to quiet him down), and YIKES, did I ever have a screamer on my hands! It took quite awhile to undo it too, and you have to be able to put up with LOTS of screaming when you're trying to correct it!
   Do keep in mind that it's normal for a cockatiel to be loud sometimes. It's not a "screaming problem" when your bird is doing it in the morning, evening, or when you first come home. That's just his way of greeting the day, greeting you, and saying goodnight.
   Remember, just because a bird's vocalizations may annoy you doesn't mean they're excessive. Keep in mind birds are NOT quiet pets, and happy, healthy birds will vocalize frequently throughout the day. My boys tend to sing and whistle for several hours at a crack.
   
   
2. BITERS
   If your bird likes to bite at your fingers while he's on your hand, you can try the "slight wiggle" method to distract him. This is NOT meant to frighten or throw your bird off balance. All you do is wiggle your finger a little to distract him. DO NOT TRY TO MAKE YOUR BIRD FALL!! Let me emphasize, the "wiggle" is not meant to scare him, make him lose balance, or make him fall! If he stops biting your finger, praise him, smile at him, tell him he's a good bird, etc...if he doesn't, wiggle your finger a little until he does. Rommel has the bad habit of chewing fingernails. He picked that up in the pet store. NORMALLY it doesn't hurt, but sometimes he gets too into what he's doing and it's painful. When that happens, I act suprised, wiggle my finger a little bit, and say "hey! that hurt!". 9 times out of 10, he goes back to softer nibbling, because he's not meaning to hurt me. If he doesn't stop, he gets up to 3 more wiggles. If he doesn't stop after that, he has to get off of my hand(sometimes he'll give a harder bite to let me know he wants down, rather than just flying off my hand). If he STILL tries to bite, he gets a time out so he can calm down.
   For very aggressive biters, you may want to try towelling instead. To do this, you get a clean, soft towel then first cover the bird, gently pick him up, making sure wings are against the body, and hold him on his back and just talk quietly to him(this is sometimes hard to do with a squirmy little budgie!). If he'll let you, you can try to gently pet his head while talking to him softly the whole time. Sometimes what's seen as "aggression" by humans is really your bird just being scared half to death. Birds can't say "I'm terrified!!", so they convey it by biting. Hard. Very hard. I've had budgies draw blood on me before.
   Some signs that your bird is frightened include hissing, feathers slicked to the body, in cockatiels the crest will be as flat against the head as possible, wide eyes, and sometimes a 'screeching' type noise. Be careful not to punish your bird for being frightened, that will only make the problem worse.
   Frightened birds need gentle persistence, patience, and need to be shown that they're in a loving environment where they won't be hurt, neglected, abused, or otherwise mistreated.
   The July 2001 issue of Bird Talk has a very good article on biting, why birds bite, and how to stop it. I encourage all new owners to get a subscription to this magazine, and encourage anyone with a nippy cockatiel or budgie to go out and buy this issue. There's actually a seperate article on budgie biting, in addition to the types of biting article.

I say "no" in a firm, but not yelling voice, followed with a "step up"(THE most important command!), and I set them somewhere(with the "down" command) acceptable.

If they run off or try to get back to where I didn't want them, repeat, and ignore them. No drama, birds love drama, just "No", "Step up", "down" and ignore. When they start playing where I want them to play, and are being good, then they get praised, played with and talked to.

Since my birds are in the room with the computers, sometimes I need them to be on the playstand or on one of the perches on the shelves next to the desk and NOT on the desk or climbing on me. I don't cage them when I'm at home.

When they get really unruly..usually just the youngest gets like this...she has to ladder. That's repeated "step up"s on a 'never ending ladder' of fingers. She repeates this 10 times, and "step up" is said in a disapproving tone accompanied with the 'evil eye'.
When she's done, she's a good bird, so she gets told that she's a good bird, then she can go back to playing or climbing on me.
If she misbehaves again, laddering again.

I never use the cage as a punishment spot, because they can end up learning "If I want to go back into the cage, all I need to do is bite/scream/throw pencils/run away from the hand."

In the rare cases when someone just won't settle down, or is repeatedly misbehaving, a 'time out' away from the flock is given.
My oldest used to have a bad habit of just yelling to hear himself yell..not singing, whistling, etc..screaming. I think he picked that up in the 2 years he was in a pet store.

None of the methods recommended for screamers were working with him, not even covering him, so when he'd start..and you could tell when he was just calling, and when he was going to just start on a 2 hour screaming binge...at the first scream, he got the evil eye with the words "time out!", had to step up, was put in the cat carrier, taken to the bathroom, covered, and left there with the lights off until he'd been quiet for 5 minutes.

Twice I had to do that before he stopped that awful habit.

He didn't like being away from me and the other birds, but on the other hand, Id idn't want my other birds picking up that habit...heh, the male budgie picked up his call and will do it occasionally though.

Since then when he starts to go into "scream mode", all I need to say is "Do you need a time out?" and give him "the evil eye" and that ends it right there.

Please note that I ONLY did this when he would scream..and he would scream..and scream..and scream...and scream...sometimes for 8+ hours(I left a tape recording when I went to work a few times) without a break. He would even scream when I was around; sort of like he just wanted to hear the sound of his own voice.
I believe he picked this habit up in the pet shop he was in for 2 years.
He was vet checked to make sure he wasn't screaming because he was hurt or sick.
This also doesn't work if you allow the behavior to go on and stop it after it's been happening for a few minutes...they won't understand why theyr'e suddenly taken away from the flock.

It has to happen the instant it starts to have hope of being effective.

For biting, mine like to chew on my nails...I don't like them to chew on my nails...so they get a little "earthquake" and a "no bite" when they do that, then are offered something acceptable to chew on like a toy, a piece of leather, etc..and are praised for chewing or biting that.
A lot of times, they're just 'overpreening' you, and don't realize that it hurts...if the preening starts to hurt, I usually just say "Ow! That hurts!!" and give them a hurt expression...facial expressions seem to work very well with most birds...anyway, they'll generally calm down and preen more softly then.

As far as shoulder riding, your bird should not be allowed to just run up to your shoulder at will. You should be the one who dictates when the bird sits there, and when the bird has to come down. If she wont' willingly step up onto your hand from your shoulder, she should not sit there until she will. If she runs around to te back of your shoulders where you cant' reach her, before trying to pick her up, back up against a wall so she can't run away from you and practice stepping up from that position. If she still won't step willingly onto your hand, and avoids or bites, I wouldn't recommend allowing her to ride on your shoulder until you get her attitude adjusted...remember, YOU'RE the top bird, not her, and she will take advantage of it if she thinks she's the top bird in the flock.
No matter what some idiots tell you, and I say this because I overheard some awful woman giving this as 'impromptu' advice to new owners at a pet shop, NEVER 'tap' the beak, or grab the beak and shake it to indicate that the behavior is bad. Not only can you accidently kill the bird, but doing this will only encourage aggression and biting. In the wild many parrots beak wrestle because it's fun or as a sign of affection, so you're just reinforcing the biting if you do this.