For domination games, I see some guys run up to the flag and dive onto it (into a prone position). This is probably really obvious, but I have no idea how they do it...
Black-ops added a really cool animation from going from a sprint to a prone position. In the other call of duty games, this was called a drop shot. Essentially from a sprint, mash the prone key and your character will 'hit the deck' by diving head first. It's important to note that you must previously be running, you can't dive standing still even if you hit the sprint button first.
Its really useful when taking fire, escaping grenades, or when you come face to face with an opponent and want to increase your chance of survival (they will have to aim down to hit you and most players have fairly conservative sensitivity)
Aardvark's answer pretty much explains how to dive. You should also know that while you're in-dive and for about a second or two after hitting the ground (ie. when the animation is playing), you cannot shoot or knife.