I am starting a leg sleeve (s) with Stuart Cripwell. I am getting tattoo about once about every other week and you can see what we have done so far below. For the first few pieces, I just picked stuff I liked and had it tattooed in spots I thought would fit my body well. Now, I still pick out the piece but Stuart outlines the negative space [ex: the empty slot of skin between two tattoos] on tracing paper. He then draws the piece, adding and taking away, so that it fits in the outline.
P.S. I am in the market for good set of kneecap tattoos ideas. I know there is a thread on this but was hoping for some more suggestions taking into consideration negative space, traditional style tattoos. I have been looking at Spider Webb's books of traditional American body suits and the knees are always hard to make out. From what I can see, though, it seems like suns, nautical stars/hexes, spider webs, and elephant heads were pretty common.