Meant to post something awhile ago on this for guys and gals that are having problems with water slides from old kits. This is a old not so known trick on bringing em back to life. First off...If the white is yellowed try putting your sheet of decals in a window that the sun beats into for a day or two. Often it will clear them up.
Second..if your unlucky enough to have a sheet that the decals are cracked but whole,scan em into a paint program and clean em up and reprint. (Thats a entirely different faq though)
Third.. If they are just plain old and look like they will be tough to release..hit.em.with a can of Polyurethane clear coat.(gloss) Allow to completely dry! Apply as normal. There is a manufacturer that made the decal paper that nothing works on,the backside will be either checkered or a aquamarine color. Sometimes I can get these to release with warm water and a bit.of vinegar in it..but I have to set them with lighter fluid (zippo).
I only use a Rustoleum Poly spray,I am sure others will work but never tried them. This.kinda cuts into decal sellers sales,but it might save ya a long term headache.