A lot of sites have online coupons that can be used for anything from a percentage off of your total purchase to a free item. All you need to know if how to find these coupons, and you could save a bundle. The best way that I've found to find the coupons is by going to Google and typing in the name of the store plus the word "coupon." For example, if you were shopping at Overstock you would go to Google and type in "overstock.com + coupon." Usually the first couple of results contain codes that you can type in during the checkout process.
2. Free Shipping
More and more sites are starting to offer free shipping on their items. Sometimes you have to spend a certain amount to get the free shipping, but if you need a couple of things and can get it from the same store it's definitely beneficial. For those sites that require you to spend a certain amount before getting the free shipping, you can just add an item to your cart that doesn't cost much. For example, if you had to spend $50 to get free shipping, but your item only cost $49.95, you'd want to add something small and pay $50.50. Otherwise, you would have to pay for shipping and it would cost $55 (or whatever the shipping rate is). Slickdeals has a portion of their site dedicated to Amazon.com "fillers" which are items that only cost a couple of cents. Check it out at http://slickdeals.net/slickfillers.
3. Compare Prices
Anytime you do shopping, whether it be online or in the store, you obviously want to find the right price. There are web sites out there that do the work for you. Pricegrabber.com tends to be a pretty good one. Just go to the site and type in what you want to buy. Pricegrabber will then tell you where you can find the best price. Keep in mind that shipping or other charges would affect the total price. If Pricegrabber tells you that buy.com has a product for $2 less than Amazon, that doesn't necessarily mean that buy.com is the better deal. You may get free shipping at Amazon and have to buy for shipping at the other site. In the end, you may be better off buying from Amazon.
Those are just a few ways that you can save money when shopping online this holiday season. There are others, but those are the best in my opinion. Also, as a side note, Google is offering $10 off of a $30 purchase or $20 off of a $50 purchase when you use their checkout system. It's definitely worth checking out at http://www.google.com/checkout/m.html.