I've only had two interviews so far, but no one has asked about my previous salary so far. So it may not even come up.
I second the "work-life balance" idea. That sounds perfectly diplomatic and reasonable.
Another idea might be to say something like "I'd like to focus my skills more on (blank.)" So... if customer service is important for the job you are applying for, maybe say you'd like to be more actively involved in customer service. Find out what the most important aspect of the potential job is, and say you'd like to focus more on that.
For example, years ago I taught English in Japan. First company had me teaching children and adults, driving all over the place to give lessons, on-call 12 hours a day like a doctor. Terrible schedule, horrible communication at the company, and no work-life balance. Also, I HATED teaching the adults. In the interview for the new job, I didn't say any of that. Instead, I said "I'm looking to narrow my focus to only teaching children, as I feel I excell in that area." The new company only taught children, so that was exactly what they wanted to hear. Find out what the new place wants, and say you are looking to focus more on that.