This is the easiest and most foolproof option. But reverse fleitz's chronology: first get another offer, then go talk to your boss.

Edit—To imsofuture's point below: Yes, come to your employer with another offer that you would actually take, but with a real preference to stay (otherwise, just go). Ask for a raise, and if the discussion doesn't go anywhere, describe the decision you face between more money elsewhere and a preference to stay if the money were not a differentiator.

This is not extortion, and a reasonable employer will not treat it as such. It is the way you find out what you're worth and get paid commensurately. Changing jobs is not the only way to get a raise, and it's certainly not the best way—for you or for your employer.