With 16 years experience, I can say this subject is like oil opinions among signmakers. Basically you're contaminating the surface or changing the chemical makeup of the adhesive by adding things in. On one hand people do it and the world doesn't end. On the other, I've seen decals get completely ruined(by me) by using the wrong stuff. The professional squeegees(not the ones like bondo spreaders) can be have a new edge put on them(actually, you could put a new edge on a bondo spreader too). -I bet that shop's owner wishes he knew that at about $5 a pop if he was getting good ones.
There are a few application fluids that are approved by decal manufacturers, Rapid Tac, Splash to name a couple but it would be a waste to buy them for anything other than professional use.
I suggest if it's just a one time thing, you're best off using a drop of dish washing liquid to about a quart spray bottle. You don't want it to feel soapy. You just want to be able to reposition it if you don't have it right. If it feels soapy, the decal most likely won't stick. After a while it may, but you definitely don't have a good bond.
Dry application is my preferred way to apply.