The first one to pay attention to is your image. In poker, your image is the way that other players at the table are likely to perceive you. For example, if you have been caught bluffing a couple of times already then your reputation will make it less likely for a bluff to work as you have a loose image. However, if you haven't played many hands and have only shown the nuts so far then your bluff has a high chance of working because you have a tight image. In general, you should bluff a lot more when you have a tight image and a lot less when you have a loose, aggressive image. That's not to say there aren't occasions where bluffing is the best play – regardless of image – but it would have to be a very convincing bluff to be successful if your image is in tatters!
Another key factor to a successful bluff is telling a story that makes sense. If you can convince your opponent that you have the hand you are pretending to have (whatever it might be) then they will probably throw their hand away. But if your story doesn't make sense then alarm bells will be set off in the heads of other players, and your bluff will be called.
So how do you tell a convincing story in no limit Hold'em poker? Let's say you have on a flop. You check, your opponent bets and you put in a check-raise. Ideally, your semi-bluff would work right away but your opponent calls and you go to the turn, which is . You now check and your opponent checks behind. The river is , meaning that you missed your straight draw and didn't even pick up a pair. However, if you bet big here you have told a very believable story; you were playing a flush draw aggressively on the flop, missed it on the turn and then made a big bet on the river when you hit your flush. If your opponent has a hand like A-9 he now has a very difficult decision to make and may fold. However, if the hand played out the exact same way yet this time the river is then it would be much easier to call a bet. All draws have missed and you would be unlikely to hold a 2 in your hand so the potential strong hands you could have are far fewer, making it more likely that you are bluffing.
One final thing to bear in mind is that great poker players do not bluff as much as you may imagine. Bluffing is definitely an important part of poker and can be a very valuable tool in your arsenal, but it's not as crucial as understanding other concepts like starting hand selection, value betting and bankroll management. Master these elements of poker first before you put too much emphasis on bluffing opportunities.