I have two different friends that are getting married this summer. They have both lived together for years and here's what they did.

One put a note in with the invitation that said "Because at first we lived in sin, we've got the sheets and the rubbish bin. Gifts would be swell, but we'd prefer a donation, to our wishing well."

My other friend put a note in with her reception information and she said, "Your presence at our wedding is the most meaningful gift you could give us. However, if you would like to contribute to our life together, please drop a small token into our wishing well along with a secret wish for us." That way people don't feel required to do so, and they don't feel as though they have to give a certain amount because you'll know what they gave you.

Anyways, there's a few more ideas for you.
Lisa