Instead of grapping your penis a guy can also move his penis against his hands or some object, comparable to the movement you make when you have intercourse in the missionary position.
The guy lies down on his stomach on the mattress or a pillow, pushing his penis up and down.
You should not use this way of masturbating. It seems that guys who are used to this method are prone to erectionproblems and have a harder time coming to orgasm when making love. Possibly they get used to so much pressure on the penis that "normal" ways of stimulation are not satisfying anymore when having sex with someone else. Here you can find more information on this subject .