In the olden days Fender put the decal on as the last step in finishing the headstock - so it was shiny glossy lacquer with a waterslide decal ontop of the finish - I don't think they even oversprayed the decal (which is why they get scraped off).
I use two things to help in applying decals.
First is a "setting solution", which is basically dilute acetic acid - aka - white vinegar. You brush a little onto the headstock and put the decal ontop of it - and then gently tamp down on the decal with fine artist brush to get it in position and flat on the surface. You can dab the decal with a little paper towel to get rid of extra moisture and vinegar.
Second, once the decal dries I use something called "Decal Softening Solution" - which does exactly that. You brush a little bit on the decal and then just let it sit - it may bubble up a little, but it will dry completely flush to the surface - just don't touch it while its drying AT ALL - or it may distintergrate.
I bought my little bottle of decal softening solution from Micromark years ago:
http://www.micromark.com/decal-accessories.html
After all of that - a light clear coat of gloss lacquer will greatly increase the life expectancy of the decal !