PC World — Here's a New Year's resolution that you'll want to keep: pay less for nearly everything you buy for your business. I'm talking computers, printers, tablets, hard drives, software--the works!

Easier said than done, right? Wrong. There's actually a ridiculously simple, but little known, way to save money on most online purchases: start them at a cash-rebate site.

It works like this: Suppose you're planning to order one of HP's spiffy Pavilion dm1z laptops. The base configuration starts at $399.99, and because it's a brand new model (well, newly refreshed, anyway), there are no deals to be found online.

All you have to do is head to a site like BigCrumbs. Or Ebates. Or FatWallet Cash Back. Search for HP, click "Shop Now," and then make your purchase like you normally would.

In about 90 days, you'll get back a percentage of your purchase. It might be 8 percent, or 5 percent, or even just 2.5 percent (the amount varies from store to store and rebate site to rebate site), but it's still money back.

And think about it: if you save 5 percent on a $400 purchase, that's $20. If you're buying 10 new laptops, that's $200. All for just a few extra clicks when you start your online shopping trip. Kind of a no-brainer, right?

Of course, I haven't told you the catch yet. That's because I haven't really found one. I recently used Ebates to order the aforementioned Pavilion dm1z, and everything worked like a charm. I was even able to use a promo code to get a discount on the laptop (which, granted, lowered my rebate a bit, but I still came out way ahead).

Indeed, I've become something of a cash-back convert. Although it takes a few months to get your rebates (which, depending on the site, can come in the form of PayPal or an honest-to-goodness check in the mail), the percentage points do add up. My take: why not use one?

If you've tried rebate sites and want to share your experiences, hit the comments!