I would bluff post-flop if I have an open ended straight draw or i flop 4 of the same suit. This way, if they fold you win anyways. If they call, and you hit your hand, you a) will probably win and b) will probably win a lot, because the pot is bigger and they are more committed.
(I guess this is only considered a semi-bluff, but it might still help you)
Pre-flop its all about timing when you bluff, not your cards. If players are acting strong and suddenly become weak (like people who raise most hands all just call on this one) on a certain hand pre-flop go ahead and bluff. They will probably fold because they most likely arent holding anything at all. If they dont fold, follow through with you bluffs but dont make them too big. Even if someone calls you to the river, it is highly possible that they will miss their draw and fold to a mediocre raise at the end. Do NOT look like a weak bluffer who folds to any re-raise or doesnt follow through because you will be constantly re-bluffed when they find out.
You dont really need that much in order to make a bluff work. If the pot is $5 and the flop doesnt help you, dont bluff out with a big $10 bet because either a) they will fold but would have folded to a much smaller bluff anyway or b) they will call because they hit something really good on the flop and you are now screwed.
Also make sure the number of players in the hand isnt too high. With 5 people in the pot with you, at least 1 or 2 of them probably hit a decent pair or more. Try to knock all but 1 player out of the pot pre-flop and then make a strong bluff after the flop. If they didnt hit something (which they probably didnt, especially when the flop is low cards), they will fold. If they did, you still are playing like you have a great hand and they may lay their hand down in fear of losing more on the turn and river.
If you get caught bluffing, try to bet the exact same way the next time that you catch a monster hand. People pick up on this and will think that you are probably trying to bluff again. So if you make a bluff and are caught thats not such a bad thing. Change your style and only play the good hands.
Hope that helped you out, and good luck!