The single most important thing is to make sure the decals are going on a smooth gloss finish. Otherwise silvering can, and most likely, will occur. Applying gloss over the decals is a personal preference and can be highly controversial, as Jonathan mentions, the teams really don't gloss coat their cars. I use Future or whatever it is called these days to seal the decals. It is really inert, but I have had it wrinkle decals on an occasion or two.