Five Finger Rule
My version of the five single rule states never to shop online without doing all five things:
1. Search for your item on http://shopping.google.com [About 5+ years ago, I used primarily http://shopping.com to compare prices, but Google's is pretty complete as well.]
2. Sort by price - Low to High. [Weeding out irrelevant results shouldn't be too much trouble..]
3. If there are multiple options at the lowest price, prioritize the online stores that aren't located in your state to avoid paying an additional 10% (ish) on sales tax.
4. Visit MrRebates to make sure you earn a cash rebate on your purchase. [Please use this URL so my broke self can earn a referral bonus + as of the time this article was written, you earn $5 just for joining! *psst* It's free.]
5. Check RetailMeNot for coupons and promotional codes to save on shipping or get additional discounts. [Sometimes, MrRebates won't grant you rebates if you try to combine it with other offers - read the fine print to make sure! However, they'll usually feature the same coupons on their site as well - if you shop through their link, you'll be able to use the promotional code AND still earn a cash rebate on your purchase!]