The best way I've found to apply decals us to paint the area in a clear gloss first. This gives a smooth surface for the decal to adhere to and reduces the chance of "silvering" which is that shiny look they get from tiny air bubbles trapped under the surface. Once the decal is on and dry give the whole thing a hit with dullcote or some other clear matt spray.
I'd also recommend you google Future floor wash as there are some good ways to use it with decals that give some great results. I've used it instead of clear gloss to paint the surface and also painted it over the applied decal and had no silvering at all. It's a very versatile product that has a lot of uses in modeling.