An appraiser with a background as an entrepreneur or venture capitalist is less likely to be put off by risk and more inclined to assign value to a company's growth potential.
Tailor your pitch to someone who's predisposed to understand your company. In other words, if you're in a sports business, look for an appraiser who's a sports fanatic.
The ability to speak authoritatively--and positively--about your competition shows that you're realistic about the marketplace and a good manager. Better managers get better valuations.
You may get a higher number if your company is evaluated in the context of a potential merger or acquisition, since many appraisers will factor in the strategic value of the deal.
If you've developed intellectual property, be sure that it's included in your appraisal. It may cost more, since the appraiser will have to do patent searches and so on, but it can boost the final number.