The standard method that works very well is glossing up the surface to eliminate any valleys and roughness associated with flat finishes and using decal setting solutions such as Solvaset or Microl Set and Sol. If you apply decals over a flat finish then you are asking for trouble. Sometimes the decals are small and few and may work but why take a risk in ruining the paint.
I like to carefully polish the model after painting with an old soft cotton t shirt which smoothes out the finish, then I gloss the model in preparation of the decal stage.
Another method that works perfectly to eliminate silvering or thick decals that leave a step is to brush on some Future where the decal is going and inmeadiately apply the decal over the wet Future and let set for a couple of minutes then brush on more Future on top. This eliminates the silvering specially between numbers and stenciling. To get rid of the step in a thick decal, its applied the same way but multiple Future coats are applied form the center of the decal out and fethered. Most if not all the step will go away. The model is then sprayed flat clear to kill the glossy finish.