The most powerful flush draw you can have is the nut flush draw (especially when we hold the Ace in our hand and it is not on the board). Let's say you have Ah-5h on a Th-9h-3s flop. Here you have nine clean outs – the remaining hearts – to win the hand plus there's a good chance that hitting an Ace would also give you the best hand. If this is the case, like if your opponent holds pocket Kings, then suddenly you have 12 outs or a whopping 46% chance of winning the hand.
In general, you should play nut flush draws very aggressively. By playing this way we give ourselves two ways of winning the hand – either our opponent folds right away and we take it down with just Ace-high, or we get all the chips into the middle with good odds of getting lucky. Both scenarios are fine and you really can't go wrong with playing the nut flush draw like this. The only instance where you may wish to consider slowing down is if you are up against a player who is extremely tight and only shows strength when he has the absolute nuts. Here, it may work out better just to check-call with your draw as you would usually be up against something strong like a set or two pair that will drastically reduce your chances of winning.
On other occasions the best play is to just call with a flush draw. A good example is when you are in position with a hand like 8d-7d on a Ad-Td-2c flop. Your opponent makes a continuation bet and the action is on you on the button. Raising here does have some positives – you could get your opponent to fold the weaker parts of his range, such as a Ten or even just King-high – but its better just to call for a few reasons. First, if you raise here and you happen to run into a strong hand (such as two pair or a set) then it's likely your opponent will raise again, putting you in a terrible situation. At that point you would have to be willing to risk your whole stack on a draw – that could have only around 30% equity - or fold on the flop and miss out on the chance of hitting your flush. Neither scenario feels very good!
Secondly, by just calling in position you leave yourself more options on later streets. Ideally you will hit your flush on the turn and win a big pot. Even if you miss however, your opponent could check to you, opening up a perfect bluffing opportunity. After all, when you called on the flop it didn't automatically mean you had a flush draw – you could have had a pair of Aces or even be slow playing something stronger! Difficulties only arise when your opponent fires again on the turn, especially when they bet big. By this point you probably won't be receiving the correct pot odds to continue with your draw and should fold before seeing the river.
Always carefully consider your hand, what your opponent might have and your position before deciding how to play a flush draw in No Limit Hold'em. It may be that you should play the same hand in many different ways!