Bluffing is an art but there are some basic guidelines to follow. First, NEVER bluff into a calling station. Know your opponent and figure out his (her) range. Once you have done that, think about what your range should look like. You should only bluff at boards where you have the range and nut advantage. For example: UTG +1 raises and I call on the button. The flop comes AJ5 rainbow. Who has the range advantage? UTG+1 of course. He is going to have AA, JJ and a ton of strong aces and jacks in his range whereas I won't have AA, AK, JJ in my range because I would 3-bet them preflop (most likely). I will have some jacks like AJ, KJ, QJ, JT, J9 (and possibly a few other suited jack combos) in my range but both the range and nut advantage belongs to UTG+1. Therefore, even if he checks, I am not betting whether for value or as a bluff on this board. However, same situation and the flop comes 964 with two hearts. Now the range advantage has flipped because UTG+1 may have 99, 66 and 44 in his range but now I have all of those PPs in my range plus, I have lots of other 9 combos and suited 9, 6 and 4 combos thus the nut advantage is closer to even because UTG+1 still has lots of big PPs in his range. So, if he checks, I am liable to bet (whether for value or as a bluff -- but more on that later) on this board. This will likely induce a call. If he does call, it is hard to narrow his range because he could still have PPs or high suited cards. So, you take a turn and if the board continues to run out favorably (meaning no overcards come) then you can continue bluffing -- ESPECIALLY if that 3rd heart comes.
The next part of knowing when to bluff and when to shut down is whether you have a chance to improve to the best hand. Again, take the above example but this time the flop comes 763 rainbow and you have 34ss. UTG+1 checks. This is a great spot to take a stab. You have a pair and a gutshot but UTG+1 can still have all the high pairs and a lot of big cards. the turn is the 8s. UTG+1 checks and you can safely bluff (or semi-bluff) again because your hand has improved to a pair with a flush and straight draw. Then the river bricks with another 7. UTG+1 checks again. This is where you go for it and bluff the river (pot sized.) UTG+1 is not going to know what you have and very few people are going to call off with just an overpair (ok, very few people in the real world...the fish around here bite a lot..lol). However, if the flop is the same and the turn comes K (even of spades) then it is a good time to shut down because that hits UTG+1's range.
You can apply this to any position at the table whether you are the one in EP and get called in MP or LP or by the blinds. If you are first to act, you can always bluff boards where you have range and nut advantage. Say for instance that you raise from UTG with 77 and the HJ calls you. The flop comes A62 rainbow. You make a cbet and that pot is most likely yours. But say in this instance, HJ calls. What does he have? I can tell you what he doesn't have: AA, AK or AQ. Maybe AJ but I would doubt it (unless he is a very loose player). Those
hands are ONLY in your range. The turn comes a K, you should blast again, and if he still calls (you can be certain he has an ace) then I would likely blast the river because he is going to have a hard time calling on all 3 streets with an ace that has a weak kicker.
I could talk about this all night but I will digress. If you want to know more, PM me and I will make a few posts about bluffing strategies.