When negotiating salary, I've found that the presence of a big performance bonus (for ex. between 0 and 30% of the salary) further complicates the process. Even more so in fields where performance can only be quantified with peer reviews and is perceived by the employer as a measure of "how well you work" or "how hard you try", thus completely under your control.
It is obvious that everybody's goal is to work so well to achieve the maximum allowed bonus, but in reality we all know this does not happen all the time despite the effort (otherwise the bonus would make no sense).
How can you make it clear to the potential employer that you are totally committed to work so well to grant the maximum bonus, but cannot accept an offer that reaches your minimum salary goal only when you achieve it?