But it's not impossible.
Some store policies—notably, matching a competitor's price—often go out the window on Black Friday, but there are still plenty of tricks savvy shoppers can use to eke out extra savings during this weekend's shopping frenzy.
(Read more: Early Black Friday sales let shoppers sleep in)
Pay with a store credit card. Target and Lowe's regularly knock an extra 5 percent off when shoppers pay with a store-branded credit card. Macy's sent its cardholders a pass for an extra 20 percent off most items on Black Friday; Kohl's mailed its cardholders one for 15 percent off. Shoppers applying for a new card get even bigger bonuses—15 percent off a first purchase with Gap, and 20 percent off with J.C. Penney. Just be sure to pay off the balance in full: Most store cards carry interest rates in excess of 20 percent.
Or a bonus-rewards credit card. Some credit cards offer bonus rewards for holiday shopping, said Anisha Sekar, vice president of credit and debit products for NerdWallet.com. Through the end of December, Discover is offering a 5 percent cash back bonus for online shopping; Chase has one for shopping at Amazon.com and select department stores. To snag the deals, cardholders must sign up in advance.
Considering the average shopper will spend about $550 on gifts this year, according to the National Retail Federation, the bonus would work out to an extra $27.50 in your pocket.
(Read more: 8 ways to stick to your holiday budget)
Track down coupons. Shoppers can stack sales and coupons even on Black Friday. Edgar Dworsky, founder of ConsumerWorld.org, recently received a Kohl's coupon for $10 off a purchase of $10 or more—which is valid on Black Friday. Dell has a coupon code good for $50 off an online purchase of $499 or more, excluding doorbusters and a few other items.
If you're already in-store, check the retailer's app. Some, including Target and Macy's, have exclusive deals that could yield last-minute savings. Target, for example, has a mobile coupon that can be scanned for $5 off an apparel purchase of $25 or more, and $2 off select Blu-ray movies.
Redeem rewards. Now's the time to use reward certificates accumulated through store loyalty programs. Consider redeeming credit card points, too. Chase and Citibank have partnerships with Amazon.com to pay with points—at a rate of a penny per point.
(Read more: Misfit toy sales may benefit deal-hunting shoppers)
Save receipts. Sometimes, the deals get even better after Black Friday. If that's the case, plenty of stores—including Macy's, Nordstrom and Sears—will offer one-time price adjustments if an item's price drops within a week or two of purchase, Dworsky said. Just present your receipt and proof of the new, lower price at that store or a competitor.
Some credit cards also offer refunds in the event of price drops. Citi Price Rewind will do so within 30 days of purchase; MasterCard Price Protection within 60 days; and Discover within 90 days, Sekar said. "You can sometimes use it to purchase gifts before Black Friday and get the match on Black Friday prices," she said. But there is some fine print—not all cards from MasterCard include the benefit, and in Citi's case, the price drop must be at least $25.
—By CNBC's Kelli B. Grant. Follow her on Twitter @kelligrant.