Pot size is vital, yes, but the key thing here is how much you make OTHER people bet. If you have a good hand, you want to protect it (against people with less good hands who could improve on the turn or the river). If you fear an opponent may have a flush draw, you want to give them BAD ODDS to call your bet. A flush draw has a roughly 20% chance of completing on the turn; therefore, you need to make them bet MORE THAN 20% of what's already in the pot.

Fortunately, there is an easy way of doing this: if you make a bet that's HALF the current pot size or higher, they are getting bad odds to draw (it's not exact, but it is a handy rule of thumb). The bigger a bet you make, the worse their odds are.

In a pure sense, this is a no-lose situation for you; if they fold, you wein the pot; if they bet, then the number of times they do make their flush will not pay for the number of times they don't make it.

In practice, of course, it's not that simple - you can't be totally sure what they have; you might have other draws yourself; you might continue to bet even after they make their flush, etc. This is where IMPLIED ODDS come in, which basically mean it can sometimes be worthwhile to call with bad odds when you're on a drwa because if you DO hit, you will make enough on later streets to pay for the times when you don't.

The other important concept is value betting, which is basically when you believe you're ahead and want to ensure that your opponents continue putting the maximum amount of money into the pot. Too big a bet can cause opponents to fold, thereby not maximising your profits. A lot of beginners undestand this, and end up SLOWPLAYING too much, i.e. underbetting in order to prevent opponentes folding, but in the process, allowing opponents to draw they cards they need to beat them with good odds.