If you can maintain the mount position indefinitely on your opponent, you're guaranteed to submit him or her. So how do you keep the mount? Well, here is a video we shot a while back on one of the ways to maintain the mount position when you have an opponent actively trying to escape.
As you can see, this technique can be easily practiced with or without a partner. If you need more evidence of it's effectiveness before you practice it, check out BJJ World Champion and UFC Lightweight Champ BJ Penn using it here during training: