In the hands prior to the final hand if you act first you can use Ken's rule of 2, 4, 5 when you're behind. This becomes the rule of 2 if blackjacks pay 2:1. If you're ahead in this situation you can bet the same as your approximation (guess) of what the opponent will do.
If you don't know what he'll do you can bet about half your lead. This prevents your opponent from taking the low, keeps half your lead intact if he takes, the low, and forces him to take a relatively high risk if he wants to take the high. On the second-last hand in this situation you can hold back your opponent's bankroll, plus minimum bet, plus a chip.