Be willing to negotiate in the first place
There are a lot of experienced negotiators out there. If you're buying a house or a car, or taking a new job, you can be sure you'll have to deal with such a person. If they can see you're timid about the whole business, many will take advantage of that fact.
You also shouldn't be shy about turning something that may not immediately appear to be a negotiation into one. If I'm buying a few expensive things from the same store, I'll often ask them to throw something in for free or reduce the price. Just because there's no sign saying you can do that, doesn't mean you can't. Often, simply by asking for something extra I'll get a better deal.
Don't get emotionally involved
One big mistake many amateur negotiators make is to become too emotionally attached to winning. They shout, threaten and demand to get their way. This is all counter-productive.
Most deals are only possible if both people feel they're getting something out of it. If the person across the table feels attacked, or doesn't like you, they probably won't back down. Many people hate bullies, and will be more willing to walk away from a transaction if it involves one.
Here are the keys to becoming a successful negotiator.
The Opening: This is not a begging session, nor is it demeaning to ask for what you want. It's an honorable exchange. However, don't come in making unreasonable demands. Keep in mind that this is a give and take, and keep it respectful. Signal your intention to get what you want, but be prepared to offer something in return.
Know that it is not personal: This is the undoing of any meaningful strategy in a negotiation. Being emotional has no place. Fix your mind on the goal, not on how you feel - betrayed, overlooked, left out, under-compensated, unacknowledged. Anger and resentment foreclose any deal, and if you let these emotions take control, you can actually end up with less in the end.
Do your homework: Educate yourself beforehand. All too often, we come to the table unprepared, not recognizing that negotiating is a two-way street with both sides getting something of value. We have to convince the other side with a powerful argument, and that takes preparation. You have to research what it will cost the other side and how you can make it worthwhile. Benefitting both sides, not only saves face, but also provides an on-going working relationship that is anything but all or nothing.