Without knowing a bit more about your bridesmaids it's hard to know!
One thing you might try is to get a nice silver photo-frame, and then after the wedding day you could choose a picture of you and them to go in it.
There are lots of gift ideas in the
Wedding Gift section on the site so browsing through there might throw up some good ideas - perhaps some simple jewellery (eg a silver bracelet) or you could get something like a low-cost Carriage Clock and have the back simply engraved with a simple thank-you message from you both.
Good luck - let us know what you choose!!
[Note: This message has been edited by Dave]