There is a good related article on this topic:
Dealing with Questions About Salary History by Michael Chaffers
QUESTION: How do I respond to the salary history question when I am currently grossly underpaid for the work that I do, and I am trying to correct that as I interview for new jobs?
ANSWER:Your best strategy is to keep them focused on what is an appropriate amount for you to make given your experience, skills and credentials today. This requires some homework, since you have to translate those intangibles into a dollar figure or a range of figures. In addition, be prepared to explain why you are seeking a significant jump in your salary and be ready to help the employer justify paying you this increased amount -- those people do not want to feel as if they are overpaying you. You could try saying, "I chose to work at my last job for less than my market value for very specific reasons (e.g., gain experience, restart career, they had money problems). Now that I have benefited from experience, as I look for a new employer, I want to make sure that I am being paid fairly for my talents."
As you follow this advice, do not forget that you have to be prepared to discuss your current salary, even though it ought to be irrelevant. If that information matters to the employer, they will either insist on talking about it, or they will learn it another way. Try to cover the issue quickly and steer the conversation back to its rightful place -- what you ought to make, given the value of your talents in the market.