First, you have to disguise your coverages behind the blitz, so the QB doesn't get a good pre-snap read. Second, you have to disguise the blitz itself so the O-line and QB doesn't know where it's coming from, giving the QB less time to read what is happening with the coverage post-snap.

This is what Bronco was so good at. It's also one of the strengths of the 3-4 defense. You have more people who can drop in to coverage, so you can disguise your coverage better. You also only 3 down linemen... so offense only knows where 3 guys are coming from. The other 1 or 2 rushers could come from a multitude of places.

Obviously, you have to pick your spots. Blitzing too often is worse than not blitzing often enough. Do it enough to keep the offense off balance, but not so often that they start to figure out what you're doing and can anticipate it.