Don't walk into a hotel without a reservation- once in the lobby, hotels figure you're already committed. If necessary, phone from the parking lot, and ask for their best rate. Then ask if they can do any better.
Don't buy air filters and windshield wipers at the oil change place. Changing them is not rocket science, and you can use the cash you save for another gallon of gas.
Don't put the empty box from your new big screen TV in the trash can. You might as well put a neon sign in your window announcing "expensive electronics inside". Cut the box up into strips and bag them first.
Don't sell your diamond to a jeweler without getting three bids. A jeweler's bid will reflect his need and his bank balance. If either is low, you'll get a lowball offer.
Don't buy extended warranties. If the item you're buying is so crappy you need an extended warranty, find one that isn't.
Don't engage in a complicated deal for your car. So many people drive a hard bargain for a new car, then give away the ranch with their trade-in. Treat them as separate transactions. If you have the patience, sell the used car yourself. You can save thousands, I repeat, thousands of dollars.
Don't ask your mother, your husband or the salesperson if those slacks make your butt look big. If you have to ask, you already know the answer.
Don't buy on the first day of release. Unless you live for the thrill of whipping out the latest gizmo, waiting a bit will allow the bugs to be fixed (hello, Firefox) and the supply to catch up with demand.
Don't trust your sex partner about herpes.
Don't buy anything but Girl Scout cookies sold door to door. Unless you really want a non-ending stream of salespeople at your stoop. Are you really that lonely?