[Harv72b]

Getting 10:1 or better on the river in a heads up pot, you should never be folding an overpair to a single bet (a raise is different).

Whenever you're involved in a hand, you should always be thinking about what your opponent(s) may have. Their range of possible hands...every bet, raise, or call they make should help you to narrow that range, as will developing a read on each specific opponent over time. You should also be looking at the size of the pot and, if you believe that you might be behind, how many outs you have to improve to a winner--basic pot odds stuff which I'm sure you know.

Here's where it gets fun: when you're looking at a river decision in a heads up pot, take the range of hands your opponent could have based on his play thus far, and look at how much of that range you beat. For example, in this hand you estimate he's got:

Trip fours or a full house 40% of the time.
A smaller overpair which didn't boat up 20%.
A three 5%.
A slowplayed overpair to your jacks 5%.
A flopped OESD 20%; half of those (10% total) made a straight with the 6, the other made a worse two pair with it.
Is just an idiot 10% (standard estimate at these stakes)

Against that range on the river, you lose 55% of the time. But, you're getting pot odds to call of better than 10:1, so you only need to win the pot 10% of the time for calling to be correct (that's not even factoring in the value of getting to see his hole cards when you call vs. not seeing them when you fold; improving your read). It's an automatic call.

Do the same thing on the flop & turn, only now look at the pot odds you're being offered to call down in the hand--in a heads up 6BB pot on the turn, for example, you need to put in 2 BBs to win 7 BBs at showdown, so you're getting about 3.5:1. If you figure that you beat around 35% of his range, you call down (that's grossly oversimplified, but you get the idea).

It's a ton of thinking to do in real time, but as you get used to doing it & see the same situations over and over again, it will become second nature. Most times you won't even realize you're doing it until afterwards.