Remember not to overly discuss Salary before you confirm that you have the job. If they are telling you the salary mid-interview, defer! Say "Lets focus on whether this is a good fit first before talking about money."
If they press the issue, tell them it won't matter what the salary is if the company isn't right for you. Discuss everything else, obviously come with questions of your own, then ask them if they think you would fit well in the job position if an agreement can be made on wages. If they say yes, this means you've essentially got the job, and even if you are a terrible negotiator, you won't walk away with any less.
Remember to be somewhat assertive. Like the above poster said, if they offer 20k a year, show or at least reference those wage statisics. Be sure to also reference how your experience makes you valuable to them. If you can give them a reason you will make them an extra hundred thousand, another five thousand on your salary looks like a great deal.
If it's close but not quite where you want and they won't budge because of a budget or something, ask for an extra week vacation. If you've already got them to admit that they WOULD give you the extra money but blah blah blah, an extra week instead looks like a fair compromise.
A good negotiator is someone they'll want on their team. Just controlling the situation to your advantage will make the other person feel like they've made a good hire, and that you were worth whatever extra they threw on. Just remember that whatever number they say can be magnitudes lower then what you end up getting, and it all depends on those 45 minutes in a conference room. So practice your interviewing skills and see if it makes a difference in your final offers