Not easy but this is how it's done.
Lightly tip lighter fluid (you can get this at most fag counters) into the corner of the decal you want to work. Then using a scalpel or super sharp knife lift the edge. Then pour more lighter fluid into the underside and keep lifting allowing the fluid to soak in.
Another tip is to use a hairdryer to loosen the adhesive which has a low melting point.
Lighter fluid is the best stiff though.