For context, a (closed) guard is a grappling position where one person is on his back and has his legs wrapped around the waist of the other. An open guard has one person standing and the other on his back, feet on his opponent's hips.
If you are in someone's closed guard, another useful way to pass the guard is to place your palms below your opponent's belt and push. As you push, walk your hands downwards and outwards.
There is a spot that will cause so much discomfort, your opponent's (not necessarily a male) legs will unhook. As soon as this happens, push back so he can't lock his feet again. Now you may bring one leg up onto your shoulder and grab the lapel with the same hand. Pull forward, crushing his legs into his chest, rotate 90 degrees, and go into a side mount from there.
Disadvantage
As with AlbertoGoro's method, you'll open yourself up for a triangle choke or a standard armbar. However, you are a bit further away from your opponent and thus it makes either technique more difficult.
Another technique for the big guys
If you're a fairly strong person and you're in someone's guard, it's possible to stand up, step over your opponent with one leg, then sit back down. If he unhooks his legs, you can pass the guard. If he's stupid, though, he'll try to hang on and find that you can easily dislocate his leg from your new position sitting on his kidneys.
Wonderful softlinks, guys. Real mature.