Speaking of Calvin's Clocks: Bridging the XP gap

XP is supposed to be a all about rewarding players who:

1. achieve certain goals,
2. role-play well,
-or even just-
3. show up consistently.

However, depending on the system, and just how spotty someones attendance is, XP can actually be a deterrent to consistent attendance and performance. If you miss a few sessions and the game goes on without you, other players begin to out-pace you in XP. Depending on the game, this will either be barely noticed or hugely obvious.

In that later situation, a person who's lagging behind gets delegated to the role of sidekick, or just gets overshadowed constantly. As such, they begin to enjoy the game less. This in turn causes them to re-prioritize the game a little lower on their to-do list. That results in them showing up less often, and thus lagging further behind in XP. It's a vicious cycle.

Have you ever seen this (regrettable) process in action? If so, how did you cope with it?
Do you know of any games that deal with this, or side-step entirely?