I use contact cement if the decal (not waterslide type) is still attached and curling (like a rim decal). Clean well with naptha. Use a q-tip or small paint brush to coat both surfaces. Let it get tacky before applying prressure. You can use some wax paper to push down on it.

If the decal is completely off I use photo-mount spray, which is basically the same thing in an aresol.