I'm sure Tomsamson said jpeg was quicker in a recent post but to be honest I'm not sure, I just go for png for sprites / tiles and jpg for larger bitmaps ( Purely for filesize issues rather than performance reasons ).

I know it's going to cause a load of groans, but the old Flash4 syntax is quicker than the dot syntax ( I must admit I use dot syntax )

tellTarget is a lot quicker than with

Don't move gradients around and use _alpha as little as possible.

The length of var names isn't that much of a killer. I've seen so many code snippets posted where something really vital like "score" is just shortened to "s" ( And not just locally for the sake of a loop ). Don't scrimp on var names, when you look at the code in just a weeks time to rip out a cool routine for a current project you're just going to have a headache.

Wherever possible avoid "and" and "or" in an if statement. The majority of the time one of the statements won't be valid and you are causing Flash to retrieve and test all the data every time. Use seperate if statements when possible ( Also if you've got a lot of if / else statements try and put the one most likely to be meet first. In real life that's not always possible but... )

Speaking of which
(condition ? action : else) I dont beleive is faster, I think when you compile it breaks it up to flash's native if statement..
I'll try running that through FLASM later.

For some strange reason key constants ( eg "SPACE" ) are slower than using raw keycodes.

Storing built in functions in vars can speed things up, eg
var Mpi=Math.PI;
'cause it saves 2 look-ups

Precalculate whenever you can, eg
var rad=Mpi/180;

Something else you see a lot of, and we've all done it a million times, is not use values in a smart way,eg

Nearly everyone has written a player sprite routine with a keyboard reading routine. So you check the keyboard, and move your _x and _y blah, blah, blah.
Then further down the code you do your player-2-background collision checks, where you check left/ right/ up/ down.
You already know which direction you've moved in and yet you still have to check all directions, so in effect you are checking something twice.
Store all your collision checks in seperate functions and in your keyboard routine ( After you know which key has been pressed ) set up the var collisionFunction=directionToCheckFor then further down the code just call collisionFunction();

( I hope I've explained that ok )

That's more than enough from me...

Squize.