Frankly, the missionary position is the position least likely to bring a woman to climax.
But there is a subtle adjustment you can make that can increase your chances of having an orgasm: the coital alignment technique, or CAT. Have your partner move his entire body up about two inches. Your partner's pubic bone will rest on top of yours so that the base of his penis presses on your clitoris. This position provides continuous stimulation of your clitoris during intercourse, increasing your chances of having an orgasm.