The fine print on my GCs reads:
-Not redeemable for cash
-Must present original certificate to receive goods/services
-$XX maximum service redemption value
-This certificate is good for one 60-minute session at time of purchase; dividing time into separate sessions will incur an extra charge
-Price increases since date of purchase may be payable at time of redemption
-Cancellations with less than 24 hours notice void this certificate
It's as lot to get on there, but it really helps to be able to show that it's all spelled out. Then if I do bend my "rules" a little, the client ends up feeling I've been generous.