Place your decal sheet on a window pane and the sunlight will do the trick in a few hours. Make sure the window pane is not prone to condensation..!
Other than that, try aftermarket decals.
It's a good idea to keep your decal sheets outside the boxes of your models, particularly if they are in long-term storage (loft insulation kind of storage!). A zip-lock kind of bag, kept in a 'normal' environment is probably what will keep your decals best the longer .