Just don't tell them - I never have for external positions. I say that I am looking for a salary that is commensurate with the responsibilities of the job for which I am applying. My past salary history has no bearing on that issue. I have taken jobs for a lot more than I currently made and a lot less, depending on the job and other factors, such location and reduced responsibilites, the attitude of management, how I thought I would like working with specific people, etc.. There is a lot more to a job than the salary and if you approach it in that manner, I don't think anyone would fault you for it.
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