I have had no luck with Avis coupons either, usually I rent a compact car and Avis coupons are for intermediate cars or higher. On the plus side, they still give 500 miles per rental with BA.
In my experience the most hassle free company for using coupons is Alamo, I have quite often used their one free day or $25 off coupons. An Alamo coupon can be found at:
http://www.clubtripmakers.com/coupons/alamo_free.htm