For me, it's all about impulse. If I like it, and I think I have to have it, (and I can afford it), I'll buy it and then figure out where in the world to hang it. I have also purchased things I probably couldn't afford but knew I also could not live without. It always works out in the end.
Most of the art in our home is stuff we have made ourselves:
watercolours, charcoals, acrylics and linoprints, still life, landscapes, figure drawings and ttv photography.
oh, and the odd band poster because music and art go together like gin and good memories.
I really believe that if life had to be lived without art, I would choose to never leave my house.