this is part of an article from the Tamiya website.
"After the DullCote dried, I sprayed layer of unthinned Future floor wax, by airbrush, straight onto the areas where the decals were be placed. This was a necessary step because of the thickness of the decals. The Future floor wax offers a clear built-up layer that the decal “sinks” into. Once the Future was dry, decals are applied in the normal manner and a small amount of Micro-Sol was placed over the top to ensure that the decal “snugs down” onto the surface of the model with no air bubbles trapped underneath. When the decals are dry, I check for silvering and spray another layer around the outside (carrier film) edge of the graphic. When this new layer was completely dry, I gently buff the future until there was no visible edge to the decal."
here is a link to the article
http://www.tamiyausa.com/product/plastic/tips/mmmgood.html
hope the can help you.
oh yah, personally i just try my best to cut away the excess, and that usually works. Have you tried looking into dry transfers??