Future! or Klear if you're on this side of the pond.
you need a reasonably good gloss finish, but it doesn't need to be a perfect high gloss finish. add a thin coat of future/klear where the decal will go and apply the decal while it's still wet. Always gets better results for me than any setting solution, and because it's wet you have a bit of time to move the decal into place before it sets, unlike MicroSet which tends to weld the decal to the surface pretty quickly.
I've also tried future and MicroSol together on more complex surfaces - wet future as above, decal above the future, then a bit of Sol on the decal, but be cautious with this as I've not done it this way very often - normally future alone is enough