Summarize this article in one sentence.
Do some preliminary "homework" by searching the Internet or visiting other stores to find the lowest prices for the item you want.  Printing out a counteroffer or writing down the website to show the seller during negotiation demonstrates that you've done your research and are willing to go elsewhere. Make sure you're looking at comparable products. Just because you found a used model for half the price doesn't mean you can get a new one for that amount. If you're considering a house, ask a real estate agent to provide you with a list of comparable listings and sales. Also find out how long the house has been on the market — the longer it has, the more willing the owners will be to sell it at a discount. Familiarize yourself with the product's features and how they compare to similar products. The more knowledgeable you are, the better you'll be able to judge the terms of the deal. When negotiating over expensive items, looking your best will garner more respect from the other party. This is especially true if you're negotiating over a house.#Be discreet in your negotiations. If you're purchasing a television or car in a public location, the seller won't want nearby customers to discover they can get a cheaper deal. Keep the volume down and the conversation private to get the best deals. Your earlier research should give you a good idea of how much "wiggle room" you have. Be careful not to undershoot and offend the seller. When shopping for houses, a polite offer is usually 5 to 10% below the listed price. If they don't agree to your offer, you'll need to give them a reason to change their mind. It's difficult to haggle over very expensive items simply by naming prices back and forth.  Point out your loyal customer history if you have one, or offer an additional benefit. If you're buying a car, for instance, offer to have the car serviced at their garage if they sell it to you. If you're selling a home, agree to pay for a repair. Discuss any flaws in the product, however minor. A small dent or a feature that doesn't live up to its advertisement can be reasons for a discount.  Mention dissatisfaction with some aspect of the product, such as the style or lack of associated product (e.g. a wireless keyboard for a computer). Be careful not to insult the seller, especially if they are the craftsman or designer of the product. Retailers won't have to pay a credit card fee for a cash transaction, and may pass on that discount to you. Warning: This could backfire if you're trying to buy a house. Having enough cash on hand to buy a house can signify to the seller that you're wealthy enough to afford more. Act hesitant until the seller reduces the price to an acceptable level. If the seller won't reach a price you're happy with, try these other tactics:  If you're making an offer on a house, send a polite thank you email to their counteroffer, but don't respond with a counteroffer of your own for several days. This gives them time to worry you might not be interested, and to consider lowering the price. Say you have a spouse or family member at home that's limiting your budget. You could even fake a call to them if you have to. If the price is "out of your hands" the seller might concede some ground. If all you can get is a mediocre deal, ask the seller how long they can hold that price for you. Quoting that price at other establishments could cause both them and the original one to lower their price. Reading the fine print on your real estate contract or warranty plan is always a good idea. If the final price or terms are different than you expected, immediately bring it up with the other party. You may need to haggle further.

Summary:
Do your research. Dress well. Make a low but reasonable starting offer. Give the other person a reason to change their price. Ask about available coupons, sales, or a cash discount. Continue negotiating until you agree on a price or the seller balks. Keep a close eye on the transaction.