LaRue has some good pointers for you. I have done plenty of corporate fitness in the past and the best way to go about this is to approach the HR departments of the companies you are interested in working with and go from there. It would also be a good idea to have a proposal ready in case they would like for you to submit one. If you belong to networking groups, ask around to see if anyone has contacts in those companies, because it’s always better to know someone on the inside before approaching them.
Good luck to you!