What I've seen sellers do is make them for a particular dollar amount, explain how the gift certificate will arrive (via email or snail mail), and mention that it's not valid for any other shop on Etsy. I've also heard to make sure you set up your shop to accept "Other" as a method of payment - that way you can put a super secret redemption code on each gift certificate that the recipient can enter in "Notes" at checkout. There is a blog about this on Etsy with some lovely examples, but I'm not sure exactly where... Maybe try searching the Etsy Blog? Or maybe another Etsy Success member knows it...
Hope that helps, anyway. :)