I posted my favorite one in another thread, but I'll repost it here: Check your receipt before leaving the store. If there's an overring, it'll be that much easier to get it corrected. Plus, many supermarkets will give you the item for free, refunding all of what you paid for it. This can be because a sale item rang up at the regular price, or because the shelf price was incorrect, etc.
I found a couple of good tips in this month's Good Housekeeping Magazine:
1. If your income is low (the magazine example is less than $44,000 for a family of four, but it may vary by region), you may qualify for a free mammogram/pap smear. More info at
http://www.cdc.gov/cancer/nbccedp/
2. You can get rebates on OTC pain relievers, vitamins, skin-care products, etc. at
http://www.caregiversmarketplace.com/FrameSetup.asp
Don't forget to use your AAA, AARP and other club discounts.