• Don't demand more; ask permission to negotiate. Explain that given your experience and what you'll be bringing to the table, you expected a higher starting salary. Then ask if it's possible to discuss it.
• When you start talking numbers, be specific. Don't just ask for more, suggest an exact salary or small salary range. And ask for higher than what you're willing to settle on.
• Don't forget that benefits and vacation time can be part of your negotiations as well. They may not be able to give you extra money, but maybe they can cover the cost of courses and conferences, or give you an extra week of vacation.
AOL Jobs has a few more good tips.