Be confident and explain to the employer how your skills and experience will benefit them. Also consider the full package, like paid vacation time, 401k, etc. Try not to negotiate in the first interview. Only when the company is serious to hire you then voice up. Remember when negotiating, he who answers first loses.

Hope this advice doesn't come too late. I would recommend that you do your homework first and then inform them that what your expecting would be market rate plus $X. and $X is subjective to how much of relevant experience you have. If the average salary for this position is $36k for example, and you have 5 years of relevant experience, then you should be asking for around $40k at least. Hope that helps.