Answer: It has a lot to do with being able to relax and deep breathing. Many women are too sensitive after their first orgasm to continue stimulating their vulva and clitoris, and must provide non-genital stimulation for a couple minutes prior to continuing again. Some women are perhaps limited to one orgasm, in a short period of time, simply because that is how their body functions.
You might try using a vibrator, as they can provide consistent and intense stimulation. Some women find they can experience multiple orgasms while using one. The orgasms may not occur one after another, but more than one occurs during each masturbation session.
Also try watching an erotic video to get your mind sexually aroused and involved. A book or erotic pictures may do the trick.
I recommend you get a copy of the book Sex for One by Betty Dodson. She provides a lot of helpful tips and knows much more than I about this subject. She also has a three videos that show women having multiple orgasms.
The main thing is to stay relaxed and to breathe deeply, perhaps panting like they teach during childbirth classes.