Hit Point Booster (2.5e Variant Rule)
From D&D Wiki
Players often get upset when they finally level up, only to roll a 1 for their Hit Point increase. Additionally it always seemed to me that the difference between hit point levels for the different classes overly favored fighter classes while shortchanging wizards and clerics. This optional rule was created to address both of these issues.
Hit Point Booster Rule
All classes roll the specified die for their hit point increase as per the normal rules. Should the player roll a 1, then the player keeps the 1 point as a bonus hit point and rolls again. A player can accumulate multiple extra hit points if they roll a 1 multiple times in a row.
Effect of the Rule
Fighter classes have a 1 out of 10 chance of gaining a bonus point when they level up. Wizards have a 1 in 4 chance of gaining a bonus point. The end result is that this rule has only a minor effect on the hit points of fighter classes, while providing more help to the physically weaker player classes. It also becomes impossible for any player class to only go up only 1 hit point at a level up.