Phillip's Top 5 Cautions
Don't Over Animate
There is such a thing as too much of a good thing, and gratuitous animation is a fine example. There's no harm in playing with all the bells and whistles available in Flash-I'd encourage it while you learn. Just realize that to effectively communicate an idea or tell a story (which, after all, is what animation really is for), you want to refrain from superfluous animation, which can ultimately detract from your message. For every effect you want to add, ask yourself, "Does this help clarify my message or not?"
Do You Really Need that Sound?
You should force yourself to consider each sound you use, and if it's not adding something to your file, it's most certainly distracting because it's adding to the file size. Gratuitous sound effects are worse than gratuitous visual effects because sounds add significantly to the file size. Just be extra critical when asking yourself whether a particular sound is really necessary.
Fake the Alpha Effect
The Alpha color effect will force your audience's computers to work especially hard. The message you're trying to communicate might be overlooked when the user notices everything slowing down to a crawl. I don't want to suggest that you should never tween Alpha, but it's the most processor-intensive effect available, and sometimes you can simulate the same effect in other ways. Consider tweening based on the Brightness color effect. If the background is white anyway, this is visually no different, but it doesn't slow down the computer as much.
Don't Shape Tween Mixed Lines and Fills
It's best to avoid tweening between shapes that don't have the same combination of fills and lines, because the results are unpredictable. Tweening a straight line into a bent line usually works fine. But if you try to tween from a line to a filled shape, you may get unpredictable results. Flash can tween lines, fills, and even tween a fill with a line. Flash has difficulty, however, when one keyframe has a line and the other has a fill (or when one keyframe has both line and fill and the other only has one). Flash does what it can to interpolate the in-between frames when you mix them, but eventually something has to give; Flash can't perform miracles.
The Limits of Masks
There are several things you simply cannot do with masks. Here are a few limits (and some workarounds). You can't have more than one symbol in the Mask layer. When you need multiple objects (like two holes in the mask), be sure to Modify > Break Apart everything in the Mask layer. Also, you can't combine Guides and Masks. Recall that Motion Guides involve a pair: the Guide and the Guided. Similarly, masking involves a Mask layer and a Masked layer. You can't combine them directly to have, for instance, a circle following a Motion Guide and masking the contents of the layer below it. Finally, when you place a Movie Clip symbol in a Mask layer, it actually behaves like a Graphic symbol.