1. Don't buy the accessories. Stores put really low prices on things like computers and TVs, just to get customers in the door. They make their real money on all the cables and peripherals. So buy what you came for and leave the rest behind.
2. Read the fine print and understand it. Especially for things like cars, the deal only looks good UNTIL you go through the fine print. But a lot of people don't do that until after they've made the deal.
3. Put your ego away. Some customers come in over-confident, because they've done their research. Smart salespeople will use that against you. They'll say things like, "Obviously you know what you're talking about, so let's cut to the chase."
4. Recognize a lie when you hear it. When they tell you a price is only available TODAY, it's a lie. If they can give it to you for that price today, they can do it at that price as long as it stays in inventory.
5. Never leave and then come back. You might think you're giving a salesperson time to soften up. But really you're just showing them that you've already decided you're going to buy it.
If you come back on your own, they'll come at you even harder.
6. There's probably not a minimum purchase. Sometimes you'll get offered a makeover at the cosmetics counter, but they'll tell you there's a minimum purchase. They could easily be lying, and expecting that no one will call them on it.
7. Don't fall for "the face." Don't let a salesperson ask you what you're currently using, and then react with surprise or disapproval. Salespeople will try to make you think you're doing it wrong, and then you'll be ready to buy whatever they suggest.
8. You're allowed to mix up different brands of makeup. Every cosmetics company tries to tell you that you have to use all their products together. They just do that to make you throw away your old stuff and buy their entire line.