There are plenty of published articles with specific information on negotiating a higher salary or haggling with salesmen, but I want to present seven things you'll need no matter what you're negotiating. Mastering and attaining these seven tenets will put you in a good position to negotiate just about anything:
Armed with the best resources and the greatest insight into the negotiation process, you'll be well equipped to buy things for less, make more money, and consistently walk away with the better end of the deal
1. Background Information
Walking into a negotiation blindly can ruin any chance you have at negotiating successfully. For example, if you're negotiating for a higher salary, you need to know what people with similar experience make at similar positions in similar companies. The more research you can do in advance, the better. Any statistics or knowledge you can bring up during the negotiation will put you in a better position and allow you to make more reasonable, calculated asks.
2. A Goal
You need to come up with something to settle on. For example, if you're buying an artifact from a street merchant and the price on the tag is $50, you could set a goal to talk him/her down to $40. If that's the case, you should probably go in somewhere around $30 (depending on the circumstances). Having a goal will give some foundational direction to the course of your negotiations.
3. A Plan
Prepare a plan, complete with hypothetical scenarios and contingency plans, to navigate unforeseen obstacles successfully.
For example, what if your negotiator insists that the price is firm? Will you walk away or come up with a new price? What if the negotiator scoffs at your initial offer and turns you away?
Preparing for these possibilities in advance will mitigate the element of surprise, which can put you on edge and make you more prone to impulsive, reactive decision making. You can't always prepare for everything, but you can have a general idea of how you will react to several common objections and challenges.
Walking into a negotiation with confidence instantly imbues you with more negotiating power. Your claims and requests will be taken more seriously, and you'll tend to earn more attractive offers from the other side. Earning that confidence takes time and practice, but it can be feigned easily enough. Walk tall, with your shoulders back when you enter a room, and dress appropriately for the situation (in most cases, overdressing is always better than underdressing). Speak directly and articulately, and make eye contact while doing so. Shake hands firmly and avoid filler words or "fidgeting" body language.
The clearer and more direct you are, the better.
Searching for a "win-win" scenario isn't an effective negotiation tactic; it's a form of compromise. It's far better to seek the best possible deal for yourself, regardless of the negotiator involved. It will help you stay firm in your goals, and demonstrate your determination, which will make you seem more confident and intimidating. It will also make you bolder in your requests, giving you better deals overall the more you apply this type of mentality.
Practice makes perfect, and it's the case with negotiations, too. On a wide scale, the more you negotiate, the better you're going to get, but it works on a smaller scale too. For example, if you're preparing to negotiate in a job interview, you can practice beforehand in a mirror with a couple of prepared responses, requests, or justifications for your claims. This way, when you do it for real, you'll be more confident, you'll seem more natural, and you'll avoid any awkward or compromising missteps. Just be careful not to over-practice, or you'll seem robotic and unnatural--exactly the opposite of what you want.
Finally, you'll need to walk into the negotiation with an objective perspective that applies to everything in the process. You'll need to know the objective costs of the products or positions involved, the statistics for people who have negotiated in similar circumstances, and the potential outcomes and motivations for both you and your negotiator. This goes along with the background information I mentioned in point one, as background information is only useful if you can treat it and consider it objectively. Even though you're hoping for a subjective outcome (i.e., one that works in your favor), an objective approach will help you get there.