I never give exact change. I might dole out the pennies to only get silver coins back, but never any more than that. Every couple of days or so, I transfer the change from my purse to an opaque jar on my shelf, and when it gets too full, I roll the change and put it in a box in the closet. When vacation time rolls around, this becomes my spending money, and by saving all my change for a year, I end up with $300-$500 bucks I didn't even feel like I was saving.
This works especially well for those of us who already keep our spending in check by leaving bills in the bank, and using cash instead of the bank card for brick-and-mortar store shopping.