1. Know Your Challenge.
Know the person you are negotiating with. What makes them tick? What is important to them? You should also have an idea in mind of what exactly it is that you want to get out of the deal. The lowest price you are willing to pay, for example. Know when you are going to bend and when you absolutely will not. Set yourself up beforehand. Ask yourself the questions you think the other party will ask, and know your answers. Prepare mentally as much as you can for the negotiation.
2. Do your research.
Know what your talking about. You will look better and often intimidate the other party if they know that you are knowledgeable in the subject you are talking about. This could be researching laws pertaining to your issue, finding other homes for sale in the area, anything that will help you to be informed and prepared. If you are negotiating on a product, please, by all means, research the product and know what you are talking about. If the other party thinks that you are not well informed, you will already be at a disadvantage.
3. What do you have that they want?
Everyone has some sort of leverage that they can use in a negotiation. Does the other party really want the commission from the sale? Do you have a product that is in short supply? Find out what your leverage is and don't be afraid to use it!
4. Be Professional.
Don't get angry, no matter how hostile the other person can be. Do not negotiate if the other party is being unreasonable. Make sure that you dress professionally and look the part that you want to portray. Make eye contact with the other person. Watch your body language, and don't give away too much.
5. Know When To Hold 'Em.
In other words, know when to walk away. Sometimes, the other party just will not bend to the point that you need. If you know for sure what you need out of the deal, and you are not going to get it, know when to stop the negotiations. Do not go beyond your bottom dollar, or the level that you set for yourself. Walk away, and know that you did all that you could.