Hi,
I was tired of finding expired coupons in my wallet or elsewhere (particularly if I've just already purchased the product!), so I created two lists in my RTM.

1) A classic list called COUPONS:
- When I obtain a new coupon in a store, I add a new task in this list, like this: shop name > amount, product, expiration date.
Auchan > 1€ Nestlé chocolate #COUPONS ^31/12
- if the coupon is valid in several stores: Shopping > 1€ Nestlé chocolate ^31/12 or General > 1€ Nestlé chocolate #COUPONS ^31/12
- if the coupon is a coupon code without minimum purchase amount: Zalando > 10€ QWERTY2012 woman shoes #COUPONS ^12/12/12
- if the coupon requires a minimum purchase amount: Zalando > 10€/50€ QWERTY2012 #COUPONS ^12/12/12
- if the amount is over 10€ (in particular if no minimum purchase amount required), I set priority 1 to be sure not forgive it before the expiration date: Zalando > 10€ QWERTY woman shoes !1 #COUPONS ^12/12/12

2) Then, a smart list called COUPONS TO USE:

list:COUPONS and dueWithin:"2 weeks of Monday"
This displays the expiring soon coupons to put in my wallet in order to use them before they lapse!

* the other coupons are stored where you want, grouped
* the coupons to use can be also displayed in a "THIS WEEK" (dueBefore:Monday and status:incomplete) and a "NEXT WEEK" (dueWithin:"1 week of Monday") smartlists
* if the coupon isn't used the expiration day, I delete the task and bin the "real" coupon: all is always up to date!

I hope my trick will be useful for you!