1. Communicate. Talk before sex, during sex, after sex. You can't read their mind and they can't read yours, so volunteer what you like and ask what they like. Not just the first time. Continuously.
(Obviously there's a lot of "idiot caveats" to this like "don't literally yak your way through sex" and "don't use 'communication' as an excuse to be bossy or whiny" and "don't blame your failings on your partner not giving an EXACT PLAY BY PLAY of what they wanted," but you get the gist of what I'm saying. Everything's got idiot caveats and I think it's pretty easy to figure out for yourself what they are.)
2. Remember individuality. You're not fucking a man or a woman, you're fucking Josh or Kyle or Nicole or Samantha. Don't assume you know "what guys like." It may not be all wrong and not every aspect of sexuality ("penis feels good") is a completely unique snowflake for everyone, but don't ever get the idea that you don't need to ask because you've learned how to do "a guy."
3. Be good. You're fucking someone because you like them, right? So assume good faith and act in good faith. Be nice, be generous, be forgiving. And if they don't respond in kind, be gone, because people aren't fixable. And all this absolutely applies to casual sex as well; just because you're not making any commitment doesn't mean you don't need to be nice to each other.
4. Unless it's a matter of anatomy or of explicitly communicated differences, follow the Golden Rule and fuck as you'd want to be fucked.
5. Don't be ashamed. They're fucking you because they like you, right? So show your whole body and everything you can do. You won't look dignified and you don't need to. A friend of mine once said of her boyfriend, "I'd touch any part of my body to any part of his body." That's a good attitude.
6. The erotic wedgie is not allowed.
I can't really think of anything else. I know this is a pretty dry and fluffy list, but anything I can think of that's explicit or specific violates Rule 2. The reason I'm not telling you how to please the G-spot isn't because I'm trying to be "serious" or anything, it's because I don't know how your girlfriend likes it. I could tell you how to do me, but I'm pretty sure that information would have limited application.
Hell, there's probably millions of women out there who think the erotic wedgie is hot.
EDIT: Jesus, I was being so fluffy that I forgot the most important rule, which is: don't give anybody any diseases and don't make a baby if you don't mean to. But that's an "idiot caveat," right? God, I hope it is.