I've found that ammonia is perfect, and I also use acrylic laquer thinner. Both clean out the Future with no problems - just make sure you blast the stuff through the gun BEFORE the Future hardens in the gun.
Using the laquer thinner, I've found that I have to clean my gun far less frequently than by using normal thinners. Blast it through at about 25-30 psi and it should remove anything in the gun. This shouldn't replace normal cleaning procedures, I just find that when I break down the gun now, it's usually spotless.
Cheers,
LeeTree