I'm always surprised when colleagues talk about buying something on the Internet and I ask, "Did you check Dealigg or RetailMeNot for a coupon?" and I get a blank stare in return.
These are great sites that collect coupon codes for sites around the Internet. It only takes an extra ten seconds to look up whether Lands End or Banana has a discount code/coupon that you've missed or is better than yours, and in the long run you'll save money. A no brainer.