My personal thought about this, and has been my practice throughout my career is to NEVER bring up salary/money at the interview. That isn't what the interview is for. The interview is for the purpose of the potential employee/employer to get to know each other a little bit and to find out whether the person is a good fit for the job/team and whether the company is a good fit for the person. All negotiations about money are done after you get the offer.
Once you get the offer you reallyhave to work with what you were/are making and then shoot for a percentage above that amount. A lot does depend on the specific discipline you are in, the level of experience you have, the amount of decision making that the position requires, etc.
It also depends on how good of a negotiator you are. For the most part it's the law of supply versus demand. You are no different than soybeans - if the demand for soybeans is greater than the supply the price goes up. The same is true with ITpeople. If nobody can find a person with experience with WILBUR on a IBM 370 mainframe, and you just happen to have that experience and are crazy enough to still be working with WILBUR in 2002, then they may have to pay more for your expertise.Mobility and location are other factors that affect salaries. You may find that companies in little podunk towns that have a factory may pay more than a job in a big city because they have trouble getting people to come there.
Remember in the interview, always tell the truth no matter what, if you lie it will come back and bite you. Stress your desire to serve and then back that up with action when you get the job.
hope this helps