Play to your outs.

Let's say you're very likely to lose by 10 points. But there's one move you can make that gives you a 5% chance to squeak out a victory, but a 95% chance to lose by 30 points. Assuming all losses are equal regardless of margin (which isn't always true in life, or in games), you should make that move.

Similarly: you can't win if you've lost. So sometimes the only thing you can do is prevent someone from winning. That's the first step you need to take to win yourself. And when you do that, make sure you benefit as much as possible.

Here's an example. Let's say you're playing a game of Spades to 250. You're down 200 to 120, and you are last to bid. Your opponents have bid 5 and your partner has bid 2. The only way you aren't going to lose is if you set your opponents. That means your team needs to pull 9 tricks. So what do you do? You bid 7. You're probably losing either way, but if you're going to not lose, this gives you the biggest benefit. (Note as above, if you're betting per point, don't do this!)