This one is more for the women and should be pretty self explanatory from the title. You ride your guy like a cowgirl rides a horse. Hence the phrase, save a horse, ride a cowboy. Guys aren't really in control of this one. It is a good chance to feel fucked by your woman instead of the other way around.
Key word here is "ride", not "grind" ladies. Many guys have experienced it, where their girl decides she wants to ride him but instead just grinds into him. Us guys can't feel jack when you do this. I know it feels great on your clit to grind into his pelvic bone, and if he is long enough, to feel the tip of his cock rub your cervix. Really though, this isn't "fucking" in the true sense of the word. You want to bounce, grind, rock, and slide. If you don't do enough to keep his cock interested it might not stay hard as you'd like it.
Now guys, I did say we aren't really in control of this one, and you aren't, but you can help her out. Use some of the upper body strength most men are common for. Grab her by the ass cheeks and help her move! Bouncing her whole body up and down will really light those thigh and posterior muscles up after a while. This is your chance to do something other than playing with her breasts...or God forbid, just resting your hands on her thighs, which would basically make you as useful to the sex act as a dildo suctioned to a bench.
While doing a typical cowgirl position (not reversed, as in the picture), a variation on this is to lean all the way back and steady yourself on your hands. This causes his cock to position more at your G-spot. Then you bounce up and down in short quick bursts, tapping that hard cock into your G-spot. Guys, for your part in this variation, the top of her slit will be pushed up making her clit more easily accessible. Double her pleasure by taking your wet thumb, and pointing downward, position it on her clit. When you do this your hand should be right over her pelvis. Now keeping your thumb on its target move your hand up and down as fast as she likes it while she bounces on you. You'll almost look like you are washing a window with that hand movement.
This is a great variation and I have seen several women rocket up off my cock after a while with an intense orgasm. If she pops off your cock cumming, push her over the top by a position change up or use your fingers and mouth to take her to another climax on top of that one.