Before bluffing you should pay close attention to who you will bluff. It's no use wanting to bluff a player who pays all the bets to the river, you'll only lose a stack, try to bluff against safe players, use a late position for that, if there is resistance from your opponents you should repeat them. Remember that you are not playing against idiots as many think, you are playing poker against opponents who have the same goal as you, to win stacks.