usually nothing, I just use porn. The very little amount of thought I have is usually like this
holy shit that looks hot
dat ass
CUM INSIDE HER >:O
dayumn
Sometimes though my imagination will take over. I'll think about a girl that I've met IRL. I'll think about the the approach and everything leading up to it. The conversation we'll have, how I'll make my moves. She's always 100% receptive to the smooth stuff i'll be pullin', etc etc and then at the end will usually come the foreplay and then the sex. Almost like a realistic simulation.
These don't happen too often when I'm masturbating though... I watch some incredible porn and my mind is usually focused on the chick(s) in the scene.
A lot of times I'll think about it during the day and this will make me super horny where I'll have to masturbate, thus getting out my trusty laptop