Re-use, Recycle
You do it with your trash (at least I hope you do!) so why not keep in the habit and do it with your Flash movies? Re-using your flash symbols cuts down on the file size. Why create three circles when one circle symbol and three instances (copies) of that symbol will do? If you need different colours, the Color Effects pop-up menu in the Instance Properties dialog box will allow you to modify the properties of each instance. Every new object increases the size of your file.
Too much of a good thing...
Animation is great, however, too much animation at once can leave your CPU up the bandwidth river without a paddle. Whenever possible, use tweened animations, which require less overhead than a series of keyframes. Also when developing flash movies for the web try to avoid:
Tweening numerous objects
Alpha (transparency), usually you can get away with tinting you object.