First, you must consider the number of opponents you have in the field. The more opponents you have, the weaker your top pair will be, relatively speaking. This is because the chances of someone having a better hand than you, or a powerful draw, goes up when there are more hands in play.
Second, you must consider your kicker. If your kicker is good (possibly even the top kicker), your hand is quite strong and should be bet and raised as such; the lower your kicker, the more you should be concerned about your hand strength.
Third, you must consider any obvious draws or even made hands visible on the flop. If it is possible for a player to have a straight or a flush, for instance, you must give serious consideration to the possibility that one player does indeed have such a hand (again, this is more likely if there are more players in the hand). If a straight or flush is possible, it's unlikely that a player has literally flopped one of these hands (though it is possible, in low-limit hold 'em!), but it's almost certain that at least one player has a draw to one of these hands, so you should be careful if another draw-completing card falls on the turn or river. If a flush or straight isn't actually possible but there is a flush or straight draw visible, you should play your top pair strongly but be constantly on the lookout for the draw to complete on the turn or river. If a player is consistently calling bets on the flop and/or turn, and possibly even cold calling them in weaker games or large fields, you must realistically put them on a draw and beware of any draw-completing cards on the turn or river.
Fourth, you must consider the strength of your pair itself. The higher the card you paired (the higher the top card is on the flop), the more comfortable you can be with your hand. If you flop top pair, but that pair is a ten, you need to be concerned about other players staying in with one or two overcards and possibly pairing that card on the turn or river, thus beating your pair. Players will often stay in hands with two overcards (or even just one!) in loose, passive low-limit hold 'em games, so be careful if an overcard comes on the turn. Sometimes, players will even call bets on the turn with nothing but an overcard, so you may want to be cautious even on the river. Indeed, in full low-limit games, top pairs of 8 or lower are often quite weak and always in danger of being beat by low straights. Top pairs of 4-8 are so weak that you may want to treat them as if you had flopped second pair instead.
Come on, I've got top pair! It's strong! Edit
OK, this is true. Higher top pairs (nines and higher) are always worth betting one bet with, and are often worth raising if a previous player has bet one bet. But going to three bets if you are given the option to do so should be done only rarely, and only if you have a truly strong top pair (a high pair, a good kicker, and no flush or straight possible), and going to four or more bets is simply not advisable with top pair unless either:
- you have a number of opponents and you are sure you can thin the field of draws who would have to cold call and would not do so, or
- you are up against only one or two other opponents and are sure you have them beat with your top pair top kicker.
Why not just go to four or more bets all the time with top pair? Because of two reasons:
- In limit hold 'em, you are often in loose passive games with many callers. There is always a small, but noticeable (say, 10-20%) chance that one of your opponents has already flopped two pair or a set. The more your opponents are willing to re-raise you, the more you must be concerned that they actually have you beat and you are behind, possibly fatally so.
- Secondly, but less importantly, due to the passive and loose nature of low-limit games, players with draws will often be getting proper pot odds to stay in the hand and can then make their draws. While any given one of these players is often far behind your top pair, a number of them collectively can actually begin to make it unlikely for you to win. If you have top pair with your queen (jack kicker), and the flop contains Q 7 8 with two clubs, and you opponents have: Q T, 9 T, and A K clubs, then if your opponents get a 6, ten, jack, ace, king, or club on the turn or river, you are immediately behind (possibly irreparably so).
If you still have top pair after the turn, betting and raising is usually a good strategy unless you believe one of your opponents has a set or two pair (or has made their straight or flush). If there is no straight or flush possible and your opponents don't have you beat, a bet or raise on the turn will usually get them to fold, since the bets double in size in this betting round.
The further you go in the hand, though, the weaker top pair becomes, if it remains unimproved. While it is almost always still worth a bet, it's unclear that it's worth a raise: if another player decides to lead bet at the pot, it is probably wisest to merely call on the river if you only have top pair: a new lead bettor is essentially claiming they can now beat the top pair that you were representing. If it is obvious that a player has made their draw (e.g. made a flush) and there is more than one player left in the field and other players are calling and/or raising, it may even be advisable to fold a mere top pair. The more players in the pot on the river with a flush showing, the more likely it is that at least one of them has a flush, small as it may be.