Aim low, and when you think you are low enough, go a bit lower. I have found that the best way to figure it out is to shoot lots of fish. If I go for a pretty good while between trips, I have to relearn a bit every time. Also, once they get below about 2' deep, it's just hard to get it right. The refraction is worse the deeper you go, and the fish has more time to react too.
Finding fish in really shallow water (like 1' or less), and shooting straight down at them really makes it easier!
Bisch