My current system involves an xp system...

Every game played with a character earns them two xp.  These xp can be spent in the following ways -

1) One point gives an automatic success on any dice roll in addition to any dice rolled.
2) Eight points gives a stat boost of one - no more than two stats above five.  (characters start with five stats at 3-4)
3) Five points gives a Descriptor.  Descriptors are mini-stats; "fast" "stealthy" "good-looking" - every point in a Descriptor gives an automatic bonus dice in any action involving it.

So no massive change to the system, and advancement would take a while.  But it keeps my xp-hungry players happy.

G