According to FoxBusiness.com the best answer is as follows, "You can give yourself some wiggle room on the question by saying you’re willing to negotiate on “total compensation.” You know, the perks, such as vacation days or flexible work arrangements, that might make the job more worthwhile even if the pay is lower than you want."
As a business owner, I have to say I agree with Fox. You have to remain transparent about your prior salary. And, you should do research on the salary allowances for the company you are interviewing with. If you know they pay low, leverage vacation days or work arrangements. If you know they pay high, show your value and why you are worth an increase in pay from your prior salary to your possible new position.
The worst thing you can do is toss out random numbers and show up to the interview ill-prepared. Do your research, know your bottom line and consider trading in dollars for days off or work days at home.
Happy job hunting!