If you have A-2-3-9 when the flop comes down K-Q-10, it's time to fold, period. You have no pair and no draw.
Suppose you have [9 43 "0 "El and the floP is 04949-With this flop, it will be hard for you to win the high side of the pot unless you hit two running diamonds. Accordingly, it's best to fold your hand on the flop if someone bets into you and you still have opponents behind you. You could call one bet with this hand if you're the last person to call that bet on the flop and the pot is very large.
This is so only because when you pick up the backdoor low draw (a low draw that you need two low cards in a row to make) or the flush draw on the turn, you will usually have the uncounterfeitable nut-low draw or the nut-flush draw. But never call a bet on the flop for a backdoor draw unless it's the nut backdoor draw and the pot is already very large.
Suppose you have 0 ~0 and the flop is [5HS-0-
This is the kind of flop that I like to bet out into and then decide what I really want to do later, on the basis of my opponent's betting and emotional response. Sometimes, that one bet will win the whole pot for me; at other times it will get raised and reraised and I'll be forced to fold my hand. If someone else bets out and there are players behind me yet to act, I'll fold my hand right there. After all, this is a pretty weak flop for me. I have only second pair.
If someone bets and everyone else folds, then I'll probably fold my hand right there, too. Sometimes I'll call in this position to see if a low card (to give me a low draw), a heart (to give me a flush draw), an ace (to give me two pair aces up), or a queen (to give me three queens) comes up. A three or a four would make me two pair and a low draw, but those cards may get me into trouble, because now I have to call all the way with my two pair! My thought process with this flop is, "Most of the time I want to win it with one bet, and I'll fold my hand if someone else bets."