Decal solvents are invaluable. Not only will they help set your decals into place without slipping, they help them to lay down flat with a minimum of bubbles. Best of all, they allow the decals to contour to complex curves (with practice and patience).
I use Microscale Industries' MicroSet (the blue bottle) and MicroSol (the red bottle). They both soften the decals, but work in very different ways.
MicroSet should be brushed onto the area where you want to lay the decal. It will help the decal to stick in place (you can still adjust the placement, tho'), and should also be applied to the decal after placement to set the decal into place. It does just what the name implies: it helps to set the decal in place and softens it just enough to allow it to lay onto the area nice and flat.
MicroSol can be used to remove decals, but is also used to soften the decals to allow forming them to curves. It should only be applied after the decal has thoroughly dried. Be very careful using MicroSol, as it is intended to soften the decal, and with rough handling the decal can break into pieces. Once the decal has dried (even if it hasn't formed to the surface perfectly), gently brush on the MicroSol until the entire decal is wet, then form it to the surface, carefully brushing out any bubbles or wrinkles. Then let it sit!! The decal may wrinkle and look as though you've ruined it, but it will flatten and smooth out as it dries. I've had success with it on complex surfaces that seemed impossible to decal, but it can take patience and several graduated applications. For best results, allow each to dry overnight, or use a food dehydrater to completely dry the decals before the next procedure.
After application of either, you can remove any excess or overwash with a wet brush. They clean up with water, but be sure to clean your brushes thoroughly to remove all the solution.
Apply your clear coat as usual to ensure permanent adhesion.
Good luck, and happy hunting!
Glenn