Since today is the last day most companies will guarantee that you'll get your standard shipping order by Christmas, many places are offering free shipping. Most websites will have a banner and/or code on their homepage. You can see Gina's previous post for a list of companies.
You start at their site and type in the name of the website you want to shop at. Then, you just click to the website and shop as usual. (There are similar sites out there, if you want to search for them, but those are the two I'm most familiar with and use.)
Ebates pays out when you hit $5, so I tend to prefer that. (I'm pretty sure they are also offering a free $10 gift card for new customers who make a $25 purchase. They are for popular places like Target or Home Depot.) ShopAtHome doesn't pay until you reach $20 and request a check, but sometimes the percentage is higher.
Even if you don't hit the payout now, the money stays in your account until you hit it. I consider it all a bonus and have received quite a bit back, over the years.