Your ability to successfully negotiate your postdoc offer will depend on a couple of factors. An obvious factor is the abundance and/or variety of grant dollars available to your prospective PI. This is one of the questions you must ask during the interview phase of your postdoc. A PI that has a variety of funds may be flexible enough to source for an increase in pay over the NIH postdoc scale (though many postdoc positions do not even pay up to the suggested NIH scale).
In my case, I got to know that my PI has a variety of funds ranging from NIH grants and startup funds as he just made a move to Stanford at the time. This is a good starting point.