So, I just completed my first Flash game, and got it up on Mochi: Santa's Magic Sack
The first thing I learned about Flash is that you can go from having a fully functioning 50FPS game to having something that drops to a snails pace really easy.
The best solution I've found so far is to create your game to run at 30FPS. (30 fps looks just fine as long as you adjust the speed your objects move)
Also, my game used "paste object to background" frequently. While this worked great for a while, after a few minutes everything slowed down.
Solution: create active objects instead of paste to background. (this may or may not work as well for your game)
Why this worked for me: My game scrolls in only one direction, and I can then destroy the active objects when they get out of frame. It seems that pasting objects just kept adding to the memory usage, with no way to clear the objects from memory, and eventually affected performance.
I've also noticed that music sounds fairly good at the lowest bitrate and frequency, but sound effects will have a short delay when you play them. Is there a solution for this?
Feel free to add your own flash tips.