To quote Daniel Negreanu:
"People don't bluff nearly as much as you think they do. Pros win because they play big pots with the best hand, not because they use super-advanced bluffing strategies. Pros win because they depend on the fact that you just won't believe them."
That being said, most of your bluffs will come from good position against limpers and/or few opponents OR from poor position against one opponent who you suspect is playing his position to steal the pot. If you are against several opponents in a pot and there is significant action, get out of the way unless you have a real hand or you're getting the odds
to chase a draw. You're hardly going to be successful bluffing an entire table off their hands. Bluffs from early position are risky because if you make a large bet and you get a caller or two, it's very difficult to continue.
As for bet-sizing, just stay away from weak, minimal bets, like 1/8 of a pot. Whenever somebody makes a bet like that on the flop, I almost always raise just to test them (hint hint).
My favorite "bluffs" are semi-bluffs from position. You can often get to the river for the price of one bet because the early position caller (if they call in the first place) will be reluctant to bet the turn. Sometimes your bluff will turn into a real hand on the river, and your river bet after checking the turn will be perceived as weakness and get a call some or most of the time.