Im just altering a bra i got from the ann summers shop yesterday and thought id show you how i do it. The smallest back size they had was a 32 so i need to lose 2 inches. Its a bit of a pain to do especially if you dont like sewing but i find it gives the best results.
Simply unpick the stiches from the hook and eye part. Cut out however much you need and sew the end back on. Simples