The first couple months after I joined, I changed my avatars on a weekly basis, and based it on whatever mood I was feeling at the time.
I stopped changing them so often because I felt that, not only was that difficult for me, but maybe for others too. To me, avatars are simply a way to locate and identify certain members more easily. Because of that.. I try not to change mine too often, because if I was looking for me, I'd have to keep track of my many different avatar changes.
So now, I just change them whenever I get tired of looking at 'em.