Learn to try and re-evaluate the strength of your hand as the board has additional cards added to it. The best way of gauging this is to try and calculate which stronger possible hands are possible holdings for your opponents and generally speaking the betting patterns will help you determine this.
For example, if there are two hearts on the flop and you bet and an opponent calls and then a non heart comes on the turn and you bet again and again get called, then if an opponent raises you or bets into you on the river when a third heart comes out there is a definite possibility of him having made a flush. Your opponent may not actually have a flush on the river but generally people that are calling you down without showing much strength are on a draw without a made hand which means they are drawing to a flush or straight.
The amount you bet and that gets called can also help in deciding whether or not your opponent is drawing to a flush or straight as usually the amount you bet dictates whether they already have a made hand like top pair or are on the draw. (The more you bet in relation to the pot size on the turn, the harder it is for them to call when drawing to a better hand - open-ended straight draws need around 9/2 whilst flush draws need around 4/1).
Betting is the language of poker and until you find resistance from your opponents you can assume that you have the best hand. When and if they raise you, then you can assume that they have a stronger hand. Think of betting as speaking whilst raising is shouting and re-raising is screaming that someone has a better hand.
Being the original bettor is an important aspect of poker as whenever you bet you give yourself a chance of winning a pot right there and then regardless of whether or not you have the best hand. By calling someone you essentially are saying that you have a hand that needs to improve and that you are prepared to play in the hope of catching up to an opponent who has already bet or that you are not sure that you may have the best hand.
Being the aggressor is essential for winning poker and so influential that you will often find that you cause opponents to fold stronger hands than yours. As a consequence, you also need to accept that sometimes you too will be forced to lay down a stronger hand than your opponent pure and simply because your opponent has bet first.