For starters, the lens you choose affects your depth of field. A long telephoto will provide shallow depth of field (DOF), whereas a wide lens has a much greater DOF. Not only that, but each of those types of lenses also makes a big impact on the scene's simplicity-a long lens isolating a particular subject from the scene, a wide lens incorporating the subject as well as its surroundings.
Once you've got your telephoto lens ready to go, the next step for shallow depth of field is to open up the lens to its maximum aperture-whether that's f/4, f/2 or something even bigger. (Remember, a smaller f/number equals a bigger lens opening.) Wide open lenses do an amazing job of minimizing DOF, which in turn puts the emphasis in a scene exactly where you want it. With every benefit, though, comes a tradeoff. And the tradeoff for working wide open is that shallow depth of field is usually very shallow and it's bound to make focusing tricky.
For slow-moving subjects, or if you feel like you've got a keen eye and steady hand (or at least really great autofocus skills), there's nothing better than a small f-number to put the emphasis where you want it-right on your subject and nothing else.
If you still need more ways to minimize the depth of field, don't hesitate to get closer to your subject. The closer a lens is focused, the shallower the resulting DOF will be. Think about it: when focusing at infinity a lens may be sharp from 50 feet to infinity-that's a lot of stuff in focus. When focused only a few feet from the camera the depth of field is often microscopic. The good news is that distractions become microscopic too.
So when you're looking for a great way to get dramatic shots, don't hesitate to simplify and rely on a shallow depth of field. The formula is simple too: a long lens, a maximum aperture, and a close focusing point equals simplicity and success.