I just started reading Super System by Doyle. He talks about that. He says that if you put your opponent on a pair of Js and you can't beat and you push hard, than that's not aggressive - that's stupid (his words).
The more you bluff, the more your bluffs will be called. If you bluff, you DON'T want a call. So to successfully bluff, you need to get caught once or twice at a table, but don't overdue it.
In one of the mags that I read (Bluff, or All-In) a pro was asked by a reader what to do - she was playing and because she was a female, she was underestimated and she was always called. She could never bluff because she was always called. So she asked how to successfully bluff when in that situation. It was a simple answer - then don't bluff. If you are getting called anyway, than don't bluff. A bluff is best used to set up other players to call you later when you have a hand. If they are calling you anyway, then don't bluff.
When I bluff, I'll fire at a pot, and if I meet resistence, and I have NOTHING, I'll lay it down. Unless I'm just shortstacked and have a low M, I won't push w/ a 2-9 on a bluff. I may go all-in on a semi-bluff (have like pair of 10s...but it's top pair and a flush draw). Then I'll overbet the 10s knowing I have outs if called...and maybe the 10s will be enough on their own.
Anyway - thought it was relevent. Doyle noted the diff between playing stupid and aggressive.