Top pair on the flop is not adequate to win a poker hand. For a start you must bet to protect it. At this stage in no-limit hold 'em it's accepted wisdom to bet colse to the size of the pot. This offers any opponent odds of 2-1. If he is drawing to a flush, he has a worse than 4-1 chance of hitting it on the turn. This is a good deal for you, ignoring the added complexities of later cards and rounds of betting. The bet also prices out opponents who are drawing to a flush with an overcard, when they have a 3-1 chance of making the flush or a pair good than yours.
If the flop has cards of three dissimilar suits, no flush is yet possible. Now an open-ended right is the only danger and you don't need to commit so many chips. A bet of half the pot offers an opponent 3-1 against his slightly good than 5-1 chance of completing the straight. Again, it also prices out a hand drawing with an overcard.
Pots Set
Why would you want to bet less? Well, every bet is risky. You may be facing a hand with two pair or a set, both very hard to see, when your opponent will probably raise you. Now you have to judge if his raise is a semi-bluff. Does he truly have the goods? Remember: you only have top pair, a hand unlikely to improve, and the default policy must be to let it and your chips go. So, it's as well to be productive with bet sizing. Over the policy of time it all adds up. In a tournament this could additionally be a life-or-death decision, effectively committing your whole stack to the hand.
Note that a flop with two grand cards matching one of the suits in your hand reduces the chance of an opponent completing a potential flush. In fact it too becomes a slightly good than 5-1 chance and you can bet half the pot. The great thing here is that you know it and your opponent doesn't! Now your productive bet sizing can suck him in to a gamble that is not profitable for him and this translates into profits for you.
Incidentally, betting more than the pot becomes a game of diminishing returns. You can never put adequate money in to offer evens and betting twice the pot only offers 3-2. If an opponent has a hand strong adequate to call 2-1, 3-2 isn't going to make a huge difference, especially when the implied odds of time to come betting rounds swamp this small margin.
Which brings us on to protecting your top pair on the turn. In other words this card has not completed any sure flush or right and you suspect still to be ahead. Again, accepted wisdom has it that a bet of half the pot here will price any opponent out of the hand but implied odds propose this is not so.
To keep the maths relatively simple, suppose there had been 4 big blinds (Bb) in the pot on the flop and one opponent remained. This seems like a pretty midpoint scenario. Now, on the turn, there are 12 Bbs: the former pot; your bet; and the opponent's call. If you bet half of this (6Bb) and your opponent calls again, the pot will have 24Bb on the river.
Then, disaster. The river card hits: it seems to complete a flush or an sure straight. Your opponent bets to laid out this. Do you call or fold? It depends on how much he bets.
Assume that he was drawing to a flush and so had a 4-1 chance of completing on the river. Four times out of five he will lose his turn bet of 6Bb for a net deficit of 24Bb. The fifth time needs to more than compensate for this. It's not hard: 18Bb already come from the pot and your bet on the turn. Your opponent only needs 6 more to break even. This is a quarter of the pot at the river.
If he bets this or less, you should call. He'll never make any money that way, which implies that you'll never lose any. In fact, you'll come out ahead by occasionally calling pure bluffs or hands where the bettor truly thinks he is ahead but you have him beat.
Unfortunately the river bet is rarely this small. You then have to apply a sliding scale to decide either to call or not. The bigger the bet, the less inclined you should be and the good hand you will need. It gets a tiny complicated.
Matters are so much simpler if you bet colse to the pot on the turn. Yes, we've gone back there with 12Bb in the pot. You bet colse to 12 yourself and your opponent calls. This makes 36Bb at the river, when the same disaster strikes. But now your opponent needs to make 48Bb to account for losing 12Bb four times. 24Bb come from the pot and your bet at the turn, so he needs to get other 24Bb.
This is a much bigger gamble on the river, fully 2/3 of the pot. Now you can call any bet smaller than this and come out ahead. In fact, again taking bluffs and misjudgements into account, you could probably call near pot-size bets most of the time.
If your opponent were drawing to an open-ended straight, at slightly good than 5-1 odds, he would need nearly 60Bb, which is a pot-sized bet. And with any kind of hand (say you'd improved to two pair or a set) you could probably call any bet - with your whole stack if necessary.
Betting aggressively raises the bar, quite a lot, at which you have to make calling decisions. Note that if you've identified an opponent who never bluffs, you simply don't call apparent made hands, thereby removing any potential gain he could make from this play. And don't forget to bet for value yourself against a known fish but beware of likely slow-players.
But they're a whole record to themselves.
Texas Hold 'Em Strategy - Protecting Poker Hands