This is a pretty simple one. Whenever I'm about to buy something online, I search Google for "<store name> coupon" which usually brings up, among other things worth investigating, the store's page on retailmenot.com. Often there is at least a coupon code or two listed that will give me 5-10% off my order, 15-20% if I'm lucky. Free money, and I don't have to be subscribed to their email newsletter to get it!
(speaking of which, some stores will give you a one-time discount for signing up for their newsletter. I often do this to get the discount, then unsubscribe a few days later)