Negotiate from Strength.
Negotiate when the time is right, when it's easist for you to get what you want.
Know What You Want - Set Your Negotiating Targets
Before speaking, know what you want and what compromises you're willing to make. Like a good poker player, set your own internal strategy and guidelines, and know what you are willing to risk. At the same time, though, make sure that your expectations are reasonable, based on the situation and on whether the person with whom you are negotiating has the authority to give you what you want.
Do Your Homework
Good negotiating means preparation. When the interviewer asks "Why?" you must have reasons.
Know How Far You're Willing to Go
After setting your salary target, establish how much you need or want this job. Be prepared to show the interviewer that you are a successful, aggressive applicant in demand by other firms or organizations.
Think Win/Win & Help the Interviewer
Negotiations in which one or both parties is intent on "winning" at the other person's expense invariably create an adversarial atmosphere that rarely works to anyone's advantage. Try to go to negotiation envisioning you and the other person not as adversaries but as two people on the same side working toward the same general goal. Be prepared to project a helpful attitude, one in which you show that you want to work with the interviewer, and arrive at a mutually beneficial conclusion. Think of ways before your interview to convince the interviewer that you should be getting what you want.
Be Prepared to Be Tough
Yes, help the interviewer, but be ready for a civilized fight. Beneath the polite words is a disagreement - usually couched in respectful "suggestions" or "ideas." He/She is seeking to get you for less, you are seeking more.
End the Negotiating on a Graceful Note
Regardless of how the negotiations play out, be gracious. You do not want to walk away from a job offer negotiation with a bad taste in your mouth. You also do not want the person with whom you've negotiated with the offer walking away with a negative impression of you.
Get the Offer in Writing Try to get a letter of employment that details the specifics of the offer, in particular whatever special arrangements you may have worked out during your negotiation.