I have printed some target decals for the LAH i am restoring and came across a product called DOUBLE SIDED JAC PAPER.
it is a clear adhesive that is similar to the adhesive used on stickers.
it comes with waxed paper (the paper you peel off your decals before you stick them on) on both sides.
I have also found it handy to reapply other decals that are in good condition but have lost their adhesive and fallen off.
I have some decals which i have removed from an old ramp and need to stick them back on after flame polishing the ramp.
The list of uses goes on.

It is available from SPOTLIGHT stores in the arts & crafts section and costs $2 for an A4 size sheet which is a lot cheeper than buying new decals when the old ones look fine and have just lost their adhesive.

I am going to try and use it for the ramp decals on my STNG (Alpha Beta & Delta ramps) where the decals stick to the underside of the ramp and need a clear adhesive to keep them in place, i will keep you guys informed and post some pics of the ramp decals

Hope this helps some of you.