General guidelines for determining taxability

Over the past 14 years, our distributorship has been audited for FET four times. Each time, the IRS assessed several thousand dollars of additional tax and interest. Because the amounts at stake were relatively small, we decided to pay the assessments. A few of the assessments were caused by mistakes made by our people, and we have no quarrel with those. But some of the assessments were wrong in our view. How can we prevent ourselves from being victimized by the IRS in the future?