Get hold of some SolvaSet at the LHS. Brush it on (looks like water) and let it do its thing. I'm not sure how it works, but I did a decal over a whole Stearman tail and there's not a bubble or wrinkle in the whole thing and that's over a Century 21 fabric covering.

When you put it on, don't rush it or get scared when it starts looking weird. Its just taking the decal and relieving the stress on it I think. It also doesn't dissolve the paint on the decal, so you can work it a LITTLE bit when you put it on.