Bluffing should probably not be an automatic reaction. Many times it takes a feel for a table to make me want to bluff. When doing it keep in mind that everyone else is also looking for an opportunity to bluff. Maybe you spot them bluffing in a common situation or they spot you. It only works when you get away with it, so you must use it sparingly.
Here are some times when you outright should not bluff.
A. When players expect you to.
Don't be the fish. A huge leak can be bluffing in telltale situations against players who know them. That's a common money-maker for the other players. Always consider this rule before attempting a bluff.
B. When you've been caught bluffing recently.
You've been labeled as a poor bluffer already. Ride it out. Let them forget that hand. Start rebuilding a reputation as a straight player so you can eventually try a bluff again later (and hopefully not screw it up again).
C. Against a dangerous flop.
If the flop has an Ace, chances are that someone has a pair of aces. Aces tend to make it beyond pre-flop. Also, players tend to continue to play their Aces. Don't bluff against Aces. You also wouldn't want to bluff against a flop like K,Q,9. Chances are someone has something they'll stick with.
D. Against lots of players.
Chances are that someone has something that they'll stick with. By bluffing in this situation, you just become an agent of that player. From an odds perspective, this is never worth it.
E. Against bad players.
As much as they love to bluff, they love to catch someone in a bluff. They're much more likely to "keep you honest" because they don't realize what a money-loser that is. It's much more profitable to play straight up in these games. Bluffing is only effective from a "fear" perspective in this case.
F. You just lost a big hand or have lost a series of hands.
Not only might you be on tilt, but other players will expect you to be on tilt, and will more readily call you.
G. You limped in, or are in a blind position.
You really have to evaluate the flop, but generally other players will think you have a poor hand and expect you to bluff.