I got a very good tip recently from the Money Saving Expert, regarding online shopping. He recommends firstly that people use a shopbot such as kelkoo or pricerunner to search for the cheapest deal (using more than one for larger purchases because no one shopbot checks all available shops).
Secondly, he then recommends that you sign up to a cashback site - RPoints being one of the best for people who spend under £500 a year - and if the store with the best deal is listed with them, then use their link to go through to make the purchase. This way you accumulate points which eventually gets you cashback. You can click through to the shopbots from these cashback sites too and earn more points.
Martin Lewis recommends that you only sign up to one cashback site because they don't payout until you've earned a certain amount of points, therefore if your earning is spread among several different sites, it takes much longer for you to get money back.