Jim,
I have used both of these techniques (there are probably more, but this is the limit of my experience):
1) 2-3 very light sprayed coats of spray can Krylon Acrylic Crystal Clear High Gloss over the printed decal sheet. Let it dry between coats. Once dry, apply as you would any water decal, let it dry in place. I then "dust" it with 3-5 coats of butyrate dope using my air brush. I use as thick a mixture as I can get my air brush to spray without blobs. In other words, don't use much thinner. BTW, Sig Lite Coat seems to work the best. Once you have enough dry coats, then one slightly heavier, let it dry. Then another until you get the look you want. You don't need an expensive air brush. Any one that will blow out dope and dust will work. Once I have enough dusting coats (and these are the most important part of making this work), I have used a brush over the decals.
2) Same as above but instead of the Krylon I have used straight Brodak or Sig butyrate dope, not thinned much, as my dry coats, using my air brush.
Only difference I have seen is that with the dusting coats on red colored decals, the Krylon really makes the red nice and bright. The dope tends to mute the red a bit.
Also, to make my decals, as mentioned earlier, I normally use Word doc, Power Point, or just a jpeg picture. Save it, then print out as you would a photo using the decal paper.
Times I have had problems is when I get in a hurry and try to load up the dusting coats. These decals don't like too much dope all at once.
Jim