Don’t treat them like an outsider. This is especially important when your project involves multiple players. Make sure that everyone working on the project knows who the other players are and how to reach them if needed. Do this right from the beginning and things will run smoothly and you’ll have the best possible end result.
Don’t be a recluse. After you give them the project, make sure that you maintain communication with them. In your first meeting set up a schedule of when and how you’ll stay in contact for approvals and feedback. Nothing is more frustrating that waiting unnecessarily for feedback and approval.
Don’t expect things to last forever. You’re busy and so are they. If you set clear expectations in the beginning and have a good contract, things should run fairly smoothly for you. They’re probably going to give you three iterations before a change order needs to be made. Respect their time and they’ll respect yours.