One of the most important things to do is to not spend a majority of sex thinking about "Oh my God, what if she doesn't orgasm again?" Because that can be a serious mood killer for the both of you, and the more relaxed she is, the more likely she will come.
Another thing to note is that a majority of females do not orgasm with sex alone. More often, stimulation of the clitoris will make females come. Key idea here: Focus on the clit. Ravage the clit. Even if it becomes too intense for her, keep going. The ways to tell if she is near climax is if the walls of the labia begin to swell and become redder.
In terms of G-spot orgasms, I've heard reverse cowgirl is the best bet, but vibrators specifically designed for targeting the G-spot can be effective as well.