bluffing is 50% art, and 50% science and takes quite a bit of practice and focus to master.
#1 most important question is whether or not your villain is capable of folding. some players just won't let go of anything. either they are suspicious that everyone is bluffing them; or they just aren't thinking on a deep enough level to even consider what YOUR bets might mean about your hand strength. either way; if you are facing one of these guys you just never bluff. period. don't bluff a calling station.
#2 This is the "art" portion of bluffing. you are telling a story, and your story must make sense. You can't just throw a bunch of money at the pot and think "I'm bluffing" and then hope it works. So, to tell that story you must imagine a hand that you want the villain to believe you have....then pretend those are your actual 2 cards and play the hand the exact way you would play it if you really had it. So, if you really turned a flush would you bet the pot on the turn and the river? Maybe you would, I don't know. But I would normally bet something like 50-60% of the pot on the turn and 40-50% on the river.
#3 Is your opponent capable of sniffing out a bluff? Is he capable of hero calling? some players are easy to bluff because they just play their own hand strength and they think things like "2nd pair isn't good enough to put my whole stack in with" or they think things like "I know he's bluffing, but I just can't call with bottom pair". These types of players you should bluff relentlessly. Other players are the opposite and are notoriously good at picking off or even inducing bluffs. You should probably not try to bluff these guys even IF you think you are telling the perfect story about your hand.
And finally...there is an old saying about bluffing.
If you never get caught bluffing, you don't bluff enough. If you frequently get caught bluffing, you bluff too much. Getting caught sometimes is part of the game...and it's good advertising for future action when you want it.