I've always found that the key to running long term games
is to get he players to invest in changing the setting, rather
than their character sheets. Give them a personal empire to
carve out and defend, a criminal enterprise to build (and
defend), a ship to keep flyin', rivals to show up, villains to
vanquish, and so forth. Something that's bigger and more
personally important than just a mission or a tavern job.

Most games already feature these things and there's no
reason they can't be the focus of "advancement."

If you'd like more specific suggestions, tell us a little more
about your game :)

L8r, --Dan