I do a ton of online shopping and whenever I do I make sure that I am getting cash back from one of several rebate sites. Most sites give you $5 in your account for simply signing up. Everyone should at least take advantage of this free $5 and cash out at least once from each site. You're halfway to cashing out as soon as you sign up. If I'm shopping at the Gap for instance, I will check each site to see who offers the biggest cash back percentage and shop through them. If I had to pick a favorite, I would have to say Ebates. I've been with them since 2003 and they now offer 2% back on eBay. Most sites give you a choice between a check or Paypal for payment.
Here's the list of rebate shopping sites that I actively use.