I don't really know either, but I do read the back of my receipts, too. A few thoughts...
Cross-promote with retailers close by. If I am grocery shopping I might run into the GNC next door if I get a $ or % off coupon (and vice versa). Or - and I've done this - when my movie stub receipt has a discount for Subway printed on it, I walk across the parking lot and eat at Subway.
Offer some deal on next visit. If I present my car wash receipt again within 7 days I get $3.00 off next visit. If I come back within 10 days I get $2.00 off. It helps build repeat visits and loyalty.
I may just be a coupon freak, but those worked on me.