I've been playing about with decals with varying levels of success, but I decided I'd use an old technique to make a sign like this:

Not sure if I've mentioned it before, or if everyone just knows about it, but it bears repeating I think.
Just take a standard inklet print out, cut it to size and also cut a piece of plasticard to the same size. Use liquid cement, like plastiweld, to glue the print out to the piece of plastic. Allow this to dry completely them add some more to the print side of the sign. This, for a reason I don't know, makes the whole thing waterproof and away you go.