I have been doing this for a while, and have found that you need to fine tune to the printer you use
White decal paper has a thick base, which will give a lumpy result
Clear decal paper must go over a very pure white surface. Printer inks are very transparent, so the decals simply won't show unless the base is white
I did try a laser print at kinkos – it smudged, but as I say, results can depend on the exact printer/paper combination. The other problem with laser is that it can't feed something smaller than a full sheet, so you end up with a whole sheet used, however many decals you print
Most printers will print at a good enough resolution to make great decals. You will have to select the highest resolution, how you do that depends on the printer menu (driver) usually selecting high resolution photo does the trick, but there are several options on my printer
The ink in inkjet printers varies a lot in how stable it is. Off brand ink is pretty much guaranteed to fade. I have an Epson 6000 that claims very stable inks, but usually pigments are more stable
The last trick is to print out your decals on paper first and cut out a couple. Check for exact size and whether they look right. Sometimes you need to simplify the image for it to look good, particularly on 15mm
John