Do you regularly get vouchers through the post for Sainsburys, but you shop in Tesco? It appears as though not many people know this, but it is possible for you to take competitors coupons to Tesco - and they will accept them. Not only that, but you can use you Tesco coupon, and your comnpetitors coupon simultaneously, on the same purchase, so you save twice! My local Tesco accept Sainsbury and Asda vouchers - the type which offer something like £10 off a £50 shop are the best.
Next time you go to the supermarket give it a go, what have you got to lose?!