This is an uncertain science. There are a few things to consider:
There have been entire books written about salary negotiation; more than that, there's a whole industry churning those same books out. What I've found works well is a combination of assertiveness and empathy (if the position's been open for any length of time, the people doing the interviewing are just as sick of the process as you are!). In my job hunt earlier this year, I went into compensation discussions knowing what my skills and experience are worth, and actually turned down my current employer's first offer -- with the understanding that it was strictly about the numbers in play, and that otherwise I'd be excited to work with them. Suffice to say, things have turned out alright :)
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