A key reason for employer success in those cases was the clear language used in eligibility provisions of the employers' plan documents. First, make certain the language you use in your plan documents and plan summaries is absolutely clear in spelling out who is, and who isn't eligible for plan benefits. Pay contingent workers on a per-job basis, rather than an hourly rate. Get written acknowledgment by temporary workers that they are ineligible for company benefits. Monitor the classification practices of those in your company who have hiring authority.
Many officers have budget accountability, but little or no training in personnel matters.