Sites like ASOS and UNIDAYS are attractive because they make effective use of promo codes - including on sale items, both for members and the general public. Encouraging people to sign up to obtain access to exclusive promo codes or special sales (and to allow people to use promo codes on sale items) would definitely result in more purchases. These could be based on any number of criteria, whether it be minimum spend, a certain percentage off the total or a certain amount (e.g. $50) once a certain spend is reached. Coupling this with spend benefits throughout the year (i.e. once you spend $2000 in a calendar year you get 10% off all purchases for the rest of the year) will 200% breed loyalty to the site rather than others.