Krylon also makes a couple excellent matte and flat clears as well as a material called "workable fixatif" which drys matte clear. Which is best depends a lot on the color your going over. Olive drab looks best with Krylon Matte clear or Testes dull coat. Other colors do well with the other mentioned coatings.. or talc in Future.
I've found it is a lot easier to finish and decal your model with gloss paints then use a matte or flat clear if a non-gloss or flat appearance is desired. Decaling over flat paints is at very best a royal PITA. Silvering is a constant problem that may not be solved even with decal setting solution.