When bluffing, you have to be able to sense weakness in your opponent, and say you raise pre flop with nothing, and bet the flop with nothing, and you get a flat call, then you are either being trapped or they are fishing for outs.
When the turn comes, and you still don't have a hand, bet a bet big enough to make them fold, but make it look liek you WANT a call, say for example a $700 pot. You bet about $400, they will either clear out, or re-raise you. Then you are up the old "sh*t creek without a paddle" and then, you should stop bluffing. Or of course they could be bluffing and you re-raise and they clear out then, but then if you are re-re-raised then you clear out......its very complicated lol
But basically, be consistant and aggressive when bluffing, you can't get scared after a call on the flop, or even the turn in some cases, if they are waiting for a spade on the river, if there aint a drawing hand out there that could possibly be hit on the river then bet again.