I actually get asked this quite a bit, but oddly enough, I've always used the rule given in the DMG.Breila wrote: I don't remember exactly which system you use for critical hits...
When a critical hit is made, damage is rolled twice. However, any modifiers are only counted once. So if you would normally deal 1d4+2 damage, on a critical you'd roll 2d4+2 damage (not 2d4+4; the modifier is not doubled).
However, a lot of people roll their attack and damage at the same time (meaning if they miss, their damage roll goes to waste). Consequently, they don't realize they've made a critical hit until the damage roll is already made, so they just x2 the result. Not actually the rule, but I usually accept it since it'd be a pain for me to roll damage for them again.