Counter target spell, then proliferate. (You choose any number of permanents and/or players with counters on them, then give each another counter of a kind already there.)
6/1/2011 You must be able to target another spell in order to cast Fuel for the Cause. You can’t just cast it in order to proliferate, and it can’t target itself.
6/1/2011 If the spell is an illegal target when Fuel for the Cause tries to resolve, it won’t resolve. You won’t proliferate.