What are some tricks for getting a woman to achieve an orgasm?

I've been with some girls who had vaginal orgasms often and some who just would never have them. Here are a few things that seemed to help:

1) Lots of foreplay. The more she gets turned on and horny, the better chance of her orgasming.
2) Finger her or use toys during foreplay. Try fingering her towards the top of her vagina and with your fingers make a 'come here' motion. At the same time press your hand down on her stomach. Girls I've done this to love this move.
3) Have her get on top. That way she can control what spots your penis is hitting.

I'm no sexpert, but for some girls it just doesn't happen. One of my current friends with benefits is like this, but it's a problem we get around with lots of foreplay and using a vibrator on her clit. This is also where spooning position comes in HANDy, cause you can use your hand to rub her clit during sex.

As far as vaginal orgasms, try different angles from standard positions. Eg. instead of regular missionary try putting her legs on your shoulders so your penis rubs along the top of her vagina and hits her G spot. Or, as mike said, let her get on top and position things as she pleases.

My girlfriend will do it everytime if I put my fingure on her clit while she riding me. It hits it when she goes down and without fail she always comes. Some lube works too because sometime when her clit gets dry takes a lot longer. Which that's not so bad for me but she gets fed up. But I think it's the secret to this whole thing.