I'm 18 so im at the peek of my sexuality, not in a relationship or anything just having fun with a few girls at a time.
I like to think foreplay starts with the second i make eye contact. I always turn things up during conversations and stuff until it moves to physical things like play fighting and gentle wrestling in my bed or even the occasional pillow fight. I find if you keep a childish mind set (like kids, on the play ground, playing cooties games and such) then things move into intense kissing and massaging, then of course oral stuff. I always tease during foreplay and make it last as long as i can.
A great thing i do is i stop right before we are about to have sex and end it there (ohh my god its so hard to do this) so the next time she is crazy for me, trust me this makes sex 10x better. I spend a good 2 hours on foreplay if i have the house to myself, if not its drasicly shortened to as much time as i have.
There is a great book out there called Sex Secrets by David DeAngelo, it comes free with his Double Your Dating book. His main point is anticipation, if you build this as high as she can stand she will end up throwing herself on you without you saying or doing anything.
Sorry to plug something you have to pay for but David has some great material.