Yeah, I know what you mean. My 30 yr old decals, both Champ and MicroScale, just go to pieces.
1. Testors Glosscoat sprayed THIN over the whole sheet. Any clear gloss lacquer would work, but not the latex versions.
2. Microscale has a decal restorer. Brush a coat on BEFORE you cut out the decal, or you get a nicely sealed decal that will NOT come off the paper no matter how long it soaks. Don't ask me how I know that.
I have tried both methods on decals going on fairly flat surfaces and have no problems with the extra thickness. I have not tried them on scribed wood, etc that needs the extra snuggling down, but if you put on a THIN coat (and THIN is all you need) there shound be no problem.)
Phil