And since it's legal to ask, I don't think there's an artful way to dodge the question. You're best to just give them an honest answer.
But ... pause here for dramatic effect ... you shouldn't stop after giving that answer. Quickly pivot to information that makes it clear you were outperforming your previous salary. Give some concrete examples of the skills you bring to the table.
You're not in a salary negotiation; you're in an interview. You need to sell yourself to the company before you worry about what you'll get paid.
Convince the interviewer that you're the most qualified candidate AND that you're a good, likable person who will fit in well with the company's culture. The time for talking money is after you've left an indelible impression on the interviewer and he or she calls a few days later and offers you the job.
If a company's final offer still seems low, consider these two things:
Is the job good enough to take the lower pay? (There's value in being happy.)
Does it present room for growth and a chance to increase your pay? (Nothing wrong with being confident that you'll impress them and move up quickly.)
That should be enough for you to make a decision, safe in the knowledge that you can always say no.
Oh, and one more thing. You might want to ask if they allow butt-Xeroxing at their holiday parties. I consider that a rather sizable perk
TALK TO REX: Ask workplace questions — anonymously or by name — and share stories with Rex Huppke at firstname.lastname@example.org, like Rex on Facebook at facebook.com/rexworkshere and find more at chicagotribune.com/ijustworkhere.