First off, some important information. Only about 30% of women are able to orgasm by intercourse alone. Here is some important information to increase the chances your partner will reach an orgasm during sex.
The average woman requires about 12 to 15 minutes of direct clitoral stimulation to be able to orgasm, which is about two to three times longer than most men are able to last. The clitoris is not located right by the vaginal opening but, in most cases, about 2.5 cm above the opening. This all plays into why intercourse may not be the most efficient way for women to orgasm.
You can increase the prospect of your sexual partner’s orgasm by using the Coital Alignment Technique (CAT), a position that emphasizes pelvic rubbing of the partner to the clitoris.
Most women orgasm easier by oral sex and direct excitation of the clitoris and the vagina. As in the classic Kama Sutra technique of using both finger(s) (for the vagina opening) and lips (suction) for the clitoris at the same time.
Other things worth mentioning: make sure her feet are warm. Subconsciously, women register cold as pain, making orgasms harder to reach. Also, many women have a hard time staying focused on body sensations and find that their mind wanders during sex. A good way to combat this is for her to play out an erotic scene in her mind. Then repeat those details over and over again until her body is more activated. Finally, make sure to take time to fully relax and get into the mood before getting started — a relaxed state is a necessary precursor for an orgasm to happen.