+1 on the drop of soap technique. That'll give you as much time as you need get the decal straight before you squeegee the water out and dab it dry.

Another good tip is to make sure the downtube decal is positioned slightly higher than the middle of the tube, so it will appear centered when viewed from above. I didn't explain that very well, but look at the first photo of Michael Angelo's beautiful Paramount above, and you'll see what I mean.