One system that I always thought was good was unlocking new skills based on prerequistes. New skills become available when existing skills reach certain levels. Thus allowing characters to progress up a skill tree of increasing complexity. This allows you to creep in terms of indivual skills but jump as new skills become available.

for example:
Level 10 Firearms - opens up the new skill sharp shooting.
Level 3 Firearms and Level 3 electronics - opens up the new skill energy weapons.