My younger son had a terrible handwriting...at 18 it is just slightly better...ironically he is a fabulous artist.....his pencil portraits look like photograghs.
I'm an artist, and my handwriting isn't all that great. People are always surprised to see my handwriting, I guess they expect it to be a work of art.

DS, on the other hand, has beautiful handwriting. So does his father. We are all lefties, so that doesn't seem to have any bearing on how good our penmanship is. I think its something you have or you don't.