I've picked up on a few thing over time and I thought I'd share what I've learned. Feel free to add your own.

1) When using the pencil tool aviod the combination of straight corners and hairline width. This often looks very amature.

2) Do not over use radial gradiant shading. Especially if it goes directly from a primary color to black, like the default ones. Radial gradiants can look great if used properly.

3) Never use Times New Roman font. IT LOOKS BORING. Be creative.

4) Up the frame rate. 12 works for beginners but you should use at least 20 if you want it to look great. It means more work but leaves a much better product. I often work around 30.

5) Change the frame size. 550 X 400 is too common. Make sure you choose your size from the start because its very difficult to change half way through the project. 400 X 300 is full screen and 555 X 300 is wide screen if you're thinking in tv ratios.

6) If you're making a stick man cartoon DO NOT MOTION TWEEN. This can't be stressed strongly enough. Its gonna create a lot of extra work for you and the finished product will not look as nice.

7) When choosing sound effects, try to avoid the ones that come packed in with flash. Everyone's heard them and it shows you didn't take the effort to find new sounds.

8) If you add in voices, a better mic does make a difference. Try to eliminate background sounds. A good way to do this is in a closet or a sleeping bag, somewhere where the sound will be insulated.

9) Use lots of layers, but make sure you label them. It may become a little crowded after a while, so use multiple scenes to address this.

Well I hope this gets you on your feet. Feel free to add your own.