I can't imagine applying a large decal any other way. Applying it dry is just flirting with disaster, especially if it's something you only do once or twice a year.
There is one method of applying decals dry that looks pretty good, but I haven't tried it.
1) First, you need to create a two-part backing so that you can remove only one side of the backing. I think that a horizontal split would work, but I've only seen it done with a vertical split.
2) Lay the decal over the plastic with the backing paper still on. Align it exactly where you want it. Take a piece of masking tape, and apply it vertically in the center of the decal and tape it to the plastic.
3) Flip up one side of the decal so that you can remove 1/2 of the backing material. If you're starting with the right side, the masking tape should be aligned just to the left of the backing split. This assumes a split that runs vertically.
4) With the right-side backing removed and held up away from the plastic, start at the centerline and slowly begin pressing the decal onto the plastic evenly, working your way from the middle to the outer edges.
5) Once the right side of the decal is completely affixed, remove the masking tape from the top of the decal, and repeat the same process on the left side. Start at the middle and work your way towards the outside edges.
6) If there are any small air bubbles, take a pin and prick a tiny hole in the center of the bubble, and press flat.
Given all that, I much prefer the total alignment control and fudge factor that the water/soap solution method affords.
Good luck...