Here's a sure-fire, though unconventional, way to prevent blisters: go barefoot.

Obviously this only work in relatively warm weather, and on most trails you'll go a bit slower barefoot, but it's the healthiest and most comfortable way to go, and you'll never get a blister this way. Although I'll admit some really rocky trails can be painful, others are a real pleasure. It also takes a little time for your feet to toughen up, and for most people it takes a bit longer for the foot and ankle muscles to develop.