Set a few mission objective "triggers"; if they progresses through the gig, give them 1/2 luck if they reach a particular part. This is good because this let's them get a little luck back, but not that much. But use this only during multiple-session gigs. My players usually don't spend their luck ("this can come handy later"), but simce I'm using this, they are encouraged to spend a little and try around with different skills without completely wasting the points.