I've had to learn some animation myself while creating game graphics, so I have a few tips:
1) Take inspiration from real life; act out the action you want to depict. Looking in the mirror helps too! If you're looking at four-legged animation, hit up some videos on youtube or find pictures. References and studying will go a long way.
2) Fast movements tend to require less frames while slower movements require more.
3) Animations with an even number of frames are used more often than not and are generally considered to be the way to go, though I could be wrong.
4) A lot of new animators will just rotate, move a few pixels, and call it a day, but it can leave a sloppy or lazy effect. Try to make actions clear and create distinct poses for each 'frame' in the sequence.
5) Exaggerate and make things lively and interesting to look at. Now, this point is subjective to what you're trying to achieve, (a solemn detective character wouldn't move with much liveliness I suppose) but generally, exaggerating movements help make things clear to the player.
Hope these few tips help if you'd like to try it yourself. In any case, I wish you luck. :]
