Traditionally, heroic progress has been the domain of levels and experience points, with each level bringing steady improvement to overshadow low initial ability scores and further differentiate the novice from the veteran. This has always been a halfway compromise, however, and few players are particularly happy when they roll up a Fighting Man with a 7 for Strength. The loophole around this, introduced in Basic Dungeons & Dragons (1981), allowed players to "buy up" prime abilities by lowering others. I've never been fully comfortable with this for several reasons, however. Take the Fighter ca. 1981, for instance. First, it made sure there was never a Fighter that started his career as a weakling. Second, it almost assured all Fighters looked the same (high Strength, low everything else) by creating "dump stats" like Intelligence and Wisdom. Third, it just felt like it undermined the random ability generation that otherwise made D&D, well... D&D.
The heroes of pulp fantasy, however, often improve in purely corporal ways. Take the young Conan, for instance, who goes from skinny whelp to brawny slayer in the course of his career. Similar stories can be found regarding wizards who go from bumbling assistants to world dominating masterminds. The following is represent this, and to resolve the aforementioned problems with low prime abilities versus "buying up," I offer the following house rule for your OD&D games:
Upon reaching a new level, a Fighting Man, Magic User or Cleric may re-roll their Strength, Intelligence or Charisma, respectively. If the new roll is greater than the previous, increase the ability to the new amount. Demi-humans and other classes gain no benefit from this rule, because they are not main characters.