The biggest mistake that people make on Black Friday is that they assume that the most popular day of the year to shop is the best day of the year to buy anything. If you're walking into a store at 5 AM Thursday morning, you're expecting floor-kissing prices in every corner. But store-wide discounts aren't in the best interest of the store. It's more common that a few tantalizing items will be sold at a loss to lure shoppers while smart floor design guides them toward more profitable (even full-priced) items. "Black Friday is about cheap stuff at cheap prices, and I mean cheap in every connotation of the word," Dan de Grandpre, a veteran deal expert, told the New York Times.

Stores know you're making this mistake, and they know how to manipulate floor traffic to their higher-margin stuff. As experts in "retail ergonomics" (it's a thing) have shown, counterclockwise traffic flows result in more spending; putting high-margin items at eye-level to the customers' right is most likely to motivate a purchase; and forcing you to walk around a display is an easy way to draw our attention to items the store wants us to throw in the cart.

(2) The Best Deals Aren't This Week (Probably).

The two most common reasons for steep discounts are price discrimination and inventory pressure. Price discrimination is the store saying: "Hey you, cheapo, I know you won't buy this steel pot at $50, so we're selling it at $40. Buy it now!" Inventory pressure is the store saying: "You didn't buy our steel pot at $50, or $40, and now it's taking up space and costing us money, so, please, just take it, how about $38?"

What's your rush? Studies of the last few years suggest that prices will stay low or go lower in early December.

It's in the stores' interest to make you think prices will go up after Black Friday. Otherwise, everybody will wait until Saturday. But as inventory piles up, prices will stay low or go lower in early December, as Stephanie Clifford has reported in the New York Times. In general, though, predicting exactly when prices on your single favorite item will be lowest is like trying to buy a plane ticket at its single lowest price. Even our smartest algorithms struggle to do it.

(3) The Full Price Is More Than What's on the Receipt.

To appreciate the net cost of your shopping trip, remember to include the gas you use commuting from mega-sale to mega-sale, the shipping and handling costs, and the warranties and rebates (much more on those later).

We tend to ignore net cost when we shop because we're focused on the bargain story. Shoppers love stories—"This skirt was 80% off, I am a discount ninja!"—because when it comes to prices, nobody knows anything, and stories are all we have. Narratives fill the space where knowledge should be. If you drive 40 minutes to a super-sale and sit in a parking-lot line for another 20 minutes, that's an hour of your time and gasoline. That hour might not be part of the story you tell yourself and your friends later. But those are real costs counting against that magnificent 80% discount you found inside.