Whether you're with a partner or on your own, try teasing. When you feel yourself building toward orgasm, shift your touch (or direct your partner to shift his; his tongue is particularly good for this) to a place that's slightly less sensitive, then move back. Even in a series, each orgasm has its own build-up, and learning to navigate increasing and decreasing sensation will help you get to higher and higher places without going over the edge. Alternately stimulating the clitoris and the G-spot can help, too, since they produce pleasurable, but different, sensations.
6. Speak up
For some women, the clitoris is hypersensitive after the first orgasm, and continued stimulation can border on painful. There's no reason to bear down and power through; tell him what you want. Betty Dodson, author of Sex For One: The Joy Of Selfloving and Orgasms For Two: The Joy Of Partner Sex recommends stopping clitoral stimulation for 10 to 60 seconds after the first orgasm, then starting again (Experiment with the timing; if you wait too long you'll start back at square one). If you fill him in on this fact—or whether you want something harder, softer, or two inches to the right—he'll most likely be happy to oblige. When men find something that works, they stick with it, so if he keeps going while you're squirming away, he probably just thinks you're writhing with pleasure.
7. Go with the flow
For some women, each orgasm in a series gets bigger and bigger; for others, each one gets smaller and smaller. It's all fine. With so much hype about multiple orgasms, part of the trick is not psyching yourself out. It's cool to have just one orgasm. It's even cool to have no orgasms, as long as you're having a really good time. Just have fun. In sex, as in life, it's as much about the journey as it is the destination.