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.