There are a few relatively simple solutions for you. In all of them, you'll want a glossy finish to apply the decals to.
1. Before you apply declas, spray the entire model with the acrylic floor cleaner called "Future". After it cures, itwill provide a nice finish for the decals to "sink into". Re-apply Future after your declas are set and then dpray a Dullcote over the entire model when that cures.
2. Use a gloss paint on the model. Apply the decals directly to the gloss paint. Seal the declas with a gloss clear (or Future) and then spray a Dullcote.
I use a lot of flat colors, too, and the Future method has always worked nicely for me. Post some pics of your work when you're done!