Glad you asked! I don't use Future as a varnish anymore since it causes some paints like Gunze to crack, I use Tamiya X-22 as a varnish. Here's the process: (1) Drop decal in water as you would usually do (2) Apply a relatively thin layer of Future to the model surface with a paint brush (3) Slide decal in place. The Future will dry pretty quickly so you don't have a whole lot of time for re-positioning. Also, Future softens the decal very aggressively (far more so than any decal solvent I have used), so after a while the decal will become so soft that it disintegrates if you try to move it. If you want to reduce the Future effect as you will want to do with large decals, thin it a little with water. If you completely screw up, fresh Future will thin dried Future so you can wipe off mistakes, but you will obviously lose the decal. If you do this, your model will initially look a bit crappy because of the very glossy areas where you applied decals, but this will disappear as you coat with flat etc.
To respond to some of the other comments above: Future dramatically softens decals, far more so than any other solution I have tried (refer to the GBU-12 pic above and note how the decal conforms to the incredibly rough surface). Future somehow creates tension in the decal such that it will be pulled down to conform to virtually any surface topography. The downside of this tension is that Future will make large decals (maybe over 2 square inches) wrinkle if not thinned down with some water. The adhesive property of Future is an additional huge bonus, I have many times accidentally applied masking tape onto a Future'd decal without it then lifting.
Thierry, Collin, I will test the Mirage 2000 decals with the Future technique once I finally get the kit. I'll take pics, and if the results are good you'll owe me a cold one.
PS@ Thierry and Colin: I like the cold one REALLY cold, as in 1 deg above freezing... which can be a bit of a challenge here in Europe...