Gosh, this has been the question for a long time. If you can get one out of ten to come in then you’ve done a great job. Most won’t know that they need it until it’s passed the time.
I think that the best thing for you to do is to simply practice what you preach. When the employees see you working out, smiling, sweating and working hard they may be challenged to come in and compete with you. The corporate environment is a very competitive place. When they see your gym as another place to compete it may be the best incentive. Put together some programs of competition and offer a small reward (month’s membership?) Who can ride the most miles, who can run the furthest, who can increase his/her bench press weight. We all love to win something.