I use water with just the slightest bit of mild dish soap in it, doesn't take much just a light mist.  Buys you plenty of time to position the decal how you want and step back and adjust if needed.  Pencil trick is key, tape it on dry, make alignment marks above and below paper every so often once you're happy with where it sits. Once I get it as best I think I can with a squeegee, I use a heat gun at a distance to speed up the drying underneath and then use a very flexible bondo spreader to go over it while still heating back and forth.  Wait several hours prior to peeling transfer paper off.  

Be careful on first pass with squeegee, decal will tend to slide on you if too much water was applied.