In the missionary position the clitoris receives only indirect stimulation, by the movement of the inner lips. There are some tricks to increase the stimulation for the girl:
If the guy lifts his torso and arches his back, the pressure on her clitoris increases. This is especially so if he moves his body forward a little after penetration, which increases the chance that the base of his penis will caress the clitoris.
If the girl tilts her pelvis upward the G-spot will receive more stimulation. This can also be done by putting a rather flat pillow under her bottom.
If the guy lifts his torso the girl can stimulate the clitoris with her fingers.
Another way to stimulate the clitoris in this position is by using the Coital Alignment Technique.