UniDays aims to bring you the best discounts for students from the best brands. ASOS are my absolute favourite for this, every so often they have 25% off with a valid NUS card for a couple of days. This website supplies a whole list of brands and companies who are working with them such as Schuh, Quiz Clothing, Burton and La Senza (with many more coming) and the discounts range from the usual 10% upto 25%
If you click on this link it will take you straight there and it takes less than 2 minutes to sign up (plus I'll get £1 for every person who is referred there from my link!
There's currently 25% off on Fashion Union so get clicking!