Ever notice how quickly the signature space on the back of your credit/debit card smears or fades beyond recognition, and how difficult it is to re-sign it? It usually happens because of moisture on the fingertips or natural skin oils.
A few months ago when I receivd a replacement card, I masked off everything but the are where the signature is signed. After signing it with a ball point pen, I sprayed two light coats of Krylon Matte Finish (suitable for plastic) over the siguature. The signature still looks like the day I signed it.
Other brands of similar sprays will certainly work, but it should be a matte finish (you can't tell it's there).
HTH