Everyone's going to say "test driven development" or "think before you write" but honestly, the only thing that has ever truly worked for me is pair coding. Just have a buddy sit over your shoulder and watch you work.

It's a bit like the integrity mechanism of developing two copies of the same software, done by totally separate teams. You and your friend are both thinking about how to solve the problem, but separately, which means the likelihood that both of you make the same dumb mistake is fairly low.