A second principle of animation, based again on human perception, is the fact that making a series of movements equidistant will not make it appear smooth to the eye. Equal distances will appear to move too quickly, then slow down, then speed up again. When distances are plotted logarithmically, they appear to the viewer to be smooth. Knowing about the logarithmic quirk of perception can help you solve this problem. If you are not using a program that figures this out, never fear.
Testing is everything.
Tweak the thing until it moves right. It just helps to know where some of the pitfalls are, so you know what kinds of things to look for. A trajectory is the path of motion you're planning to use. The key drawings are the high points of the movements, or the most telling poses. Animation of objects and effects relies more on trajectories.