It's one of the first lessons I learned when I started playing tournament poker, you can't bluff someone who doesn't realize they can lose. Most often you see this with new players who are as likely to call you down with a pair of 2s as a straight-flush - they are next to impossible to read and probably don't know what they have...
So here's a quick quiz (see if you can guess the author) on bluffing:
If you're going to bluff someone:Author? Phil Hellmuth (article can be found at The Poker Gazette). It's a good article, covering more than just bluffing. I agree with him though, there are two things that can really eat through your stack; playing marginal hands when you don't have to and bluffing against players who can't be bluffed.
a) bet enough to put some heat on him;
b) wait until you feel he cannot call you;
c) do the bluffing against someone capable of folding his hand;
d) all of the above.