IMO bluffing is very player dependant, if you're playing against calling stations or people who will gamble with any 2 cards which is fairly common at the low limits, bluffing is almost usless about 95% of the time because you'll get called down if they pair the board and/or holding just overcards. Bluffing with a chance to improve however can be more beneficial than a stone cold bluff. Generall you need to be playing against a thinking player who considers other possibilities (like the rest of the players at the table) besides his hand and the board.

Even though I don't play as much live as I do on line I think bluffs and tells play a much larger role there than on line.

First thing is to identify whether your opponent is capable of folding or not.

I note players as either

will call with nothing
will call
will fold to pressure
will fold
passive

How to identify these types is more tricky but for example if someone calls a board of AKT83 to the river with 55 they are immediately going in the second category.

There's only very rare occassions when I will bluff the first two categories and they would usually involve a continuation bet on a preflop raise with a scary flop like AQT.

Which leads to the second part. I think bluffing is often less about convincing your opponent that you have a great hand and more about working out when your opponent has nothing.

There's loads of ways to identify this, for example most common being an open raise of half pot (or bigger) from the last person to act on a checked flop. Of course there are plenty of people who will fake the bluff this way and catch you out, which is why working out who are the maniacs and who can be tricky is also key.

Basically I consider whether or not the actions of my opponent indicate that they have a hand or not, and if not (if they are capable of folding) I'll bet. I'll also factor in my table image, my actions on the hand thus far and proportionate to how likely they are to call I'll also consider my outs if they are ahead. This last part is more tricky but some massive hands can be won with a semi-bluff gone wrong that hits - not to be relied on, but a good long-term earner.

One last thing, although there's a time for bluffing the flop and turn (and even river) there's also a time to acknowledge that either the other guy has the nuts or has worked you out and let the hand go.

One cast iron rule I have is never bluff more than three people, and only bluff three very rarely. This has stood me in good stead.

Only my thoughts as they apply to MTTs and I am hyper aggressive post flop at certain stages of tournaments betting almost every flop, so others may well disagree.