On one occasion I have said my previous salary but I also said that I believe I was seriously underpaid at my previous job, so I want X + 100%. At the end I have received X + 70%.

I had very good references though -- my former student was a developer at that company, so when the boss invited him to the interview, he said "well, he taught me what I know" (which wasn't literally true, but made a good impression).