Bluffing is mandatory, but never without reason. You (and they) may bluff preflop because you have good position. The bet needs to be the same from the button if you have 25o or AKs, so they never know when you are bluffing.
Post flop bluffing is often best when you make a small pair or have some outs. If you were original raiser and the board has an Ace, you might try a bluff even if you opened with KQ. Or if you called from the blind, and the board is rags like 457, a bluff might work. I like betting flop from 1/4 to 3/4 whether a bluff or nuts.
Turn bluffs are rare unless as continuing a flop bet, and you sometimes need to bluff all three streets. You'll be surprised how often villain will call, call, fold. Rarely when an over card or third (or fourth) connector comes you can bluff. You need to bet as if you hit what you're selling.
Bluffing just the river has to make sense. Like as a continuation bluff, or when the board has four of a suit or one card to a straight. River bluffs need to be bigger, because the pot size is bigger.
You need bluffs to win, but only bluff for a reason, and have a plan if they call you out. Then you can put yourself in Villain's shoes and decide if they might be bluffing.
I agree very much with these comments especially ( does it make sense ) and ( have a reason) you should actually have a reason for everything you do at the table online or live.
Couple other things to consider ... did a better hand than yours get there .... does this person play consistently and make it easy by raising only his or her good hands or are they playing really wide ranges and in a maniac type of play .. what kind of range does someone play from that position , are they capable of playing those crappy cards so when the crappy board falls it's possible they have the goods .. how do they see your play, are you going to fold if scare cards come, I mean if you are only playing the top 10 % then they know you don't have that low straight that got there and are liable to fire .... they honestly don't need a hand to do so either ... some of those things give me clues ...
Others have mentioned a HUD I use one and it is truly indispensable, as long as it is allowed ( more and more sites seem to be going away from those, I'm sure to attract less serious players, and hopefully increase their bottom line ... so that may go away at some point, I hope not ) it will improve your game play and it will track starts for your opponents and you, so after time ( a fair sample size of hands ) it will help you make reasonable assumptions about someone else's play .. so I would say take advantage of it while you can ??
I see a lot of comments about aggression ... it is my preferred play style and it does not mean I am always bluffing .. I am learning to tone it down some and add calls and checks for balance and maybe be a little more trappy ... but in general I believe aggression is a winning style of play, and has no indication of a bluff by itself ... now you get somebody shoving on every hand .... we've all seen it ... and you know they can't have a hand every time , then I like to call more with lesser hands , someone did this to me the other night with 2-9 off and I stacked him with a pair of fives, he was shoving every hand and he built a nice little stack very quick he also lost it very quick.... one other thought if you are playing the micros to low stakes I think bluffing a lot is a mistake, you honestly have no need to balance your range ... at this level people just aren't thinking that deep, some will do it but you will see them picked off easy by everyone paying attention ...
I don't know when someone is bluffing all the time, but if they don't tell a good consistent story from the deal to the river, something isn't right, as time goes by and you increase the volume of hands you play you will get a feel when something is amiss, even then you won't always be correct ....
ok sorry for making all of your eyes bleed with my little book .. guess I had a lot I wanted to say there