Well, I've only chosen twice so far, but...
The first one is a design I'd wanted for about 10 years, and has lots of meaning to me. It encompasses a variety of Pagan spiritual symbolism - I've explained it in my intro, and in the religious tattoos thread, so I don't want to get redundant. It's on the left side of my body. Moving forward, I'm using the left side for the "meaningful/spiritual" pieces. In many Pagan traditions, the weak side is the receptive side - so I'm using it for qualities I want to draw in.
Since I'm right handed, the right side is my projetcing side. That's getting used for musical and whimsical tattoos - pretty much anything I like. Tomorrow I go in for tattoo number 2, which is going on my right arm. It's an image from a Black Crowes t-shirt I bought in the mid-90's, and have always thought it'd make a good tattoo - luckily, my artist agrees :D. Pictures will be posted when complete.
Future choices? I have some ideas, for both sides...while more meaningful stuff is in the works for the left side, I also really want a Perry the Platypus on my right leg :).