Just shop like you normally would from your online retailer of choice. When you're checking out, stop by retailmenot.com, type the name of the site in their search bar, and see if there are any available coupon codes for that site before you complete your purchase. It's completely free.
No, I'm not getting paid by retailmenot.com to say this (not that I would turn down such an offer ;)), but there are a lot of coupon code sites out there, and most of them are garbage. retailmenot.com is the only one I've found that isn't worthless.
So try it out. The worst that could happen is you'll save some money. (Actually I suppose it'd be worse if you didn't save any money, but I digress...)