When your character levels up, you have to go through a process where depending on their class and level they will get certain statistical bonuses that are randomized. By saving after your character attains enough experience points to level up but before you actually level up the character, you can manipulate your statistical bonuses, resetting the game until you get the statistical bonuses you desire.

For instance, in terms of hit points, which is based on a characters constitution, you can raise your characters hit points by waiting for huge constitution bonuses, resetting your game until that bonus is attained. This is a really useful trick to make hugely powerful parties. It just requires some patience.