If you are purchasing something online and there is a box to type in a promotion code, don't continue on your merry way. Take a moment to search the web for the name of the retailer and "coupon code".
For example, if you're ordering something on snapfish.com type "snapfish coupon code" in google (or a different search engine). This will bring up multiple sites that have the coupon codes for snapfish listed.
I did most of my Christmas shopping online this year and really saved by using this method. The site I had the most luck with was retailmenot.com. Almost every gift we purchased had a reliable coupon code on this site.