This is something I've dealt with as well. My usual strategy, when asked, is to ask what the employer plans on offering and if they push to see what I was making before I'll give them a ballpark figure of what I'd like to be making in the new position (usually what I was making before +10-20%). I've found that in the US in certain industries (such as finance) this question is almost always asked and if you refuse to answer, they'll just find out in the background check and will revoke the offer if they find too much of a disparity in the numbers. I've always been told it's better to be honest about it up front than to have them find out after the background check that you've exaggerated your numbers.