Have a clear vision in mind: You must know exactly how you want your story to be told in order to have it written just the way you envision it. The vision doesn't have to be unalterable, just solid enough to give your ghostwriter a sense of direction.
Know you own story: In order to put your story into words in a way that pleases you, you must make sure that you narrate your story and ideas to your ghostwriter in such a way that makes it easy to tell the story in a creative way without losing sight of what matters. Have as much information available (timelines, scenes, lessons etc.) ready ahead of time.
Be ready to put in some time: One of the reasons you're considering hiring a ghost writer might be due to lack of time. However, remember that during the story-writing process, your ghostwriter will need to be able to get a hold of you at different times, for various reasons. It is therefore important that you not schedule to have your project started during a stretch of time when you know you'll be hard to reach.
Communication: Be ready to communicate your needs to your ghostwriter. Again, this is your story and a reflection of you so you want to be as involved as you possibly can be. Review drafts, suggest changes, and voice concerns until you feel satisfied with the final product. Your ghostwriter is there to serve you the best way they can and you have paid for their expertise. Never feel like you can't suggest a different way of doing things.
Know that it takes time: Any piece of writing takes time. The key to finding a good ghostwriter is not to find one who promises to have your project ready in the quickest time possible, but to seek out someone with a track record of dedicating time to producing a quality product. A good ghostwriter should be able to judge how much time is needed to complete a specific project - and it almost never is in two weeks or less.
Make sure you like your ghostwriter: Never hire a ghostwriter you're uncomfortable with. If you have an odd feeling about their personality, chances are you will find their writing odd as well. Seek out someone who suits your personality, someone who gets your story and brings a certain enthusiasm to your project. It is okay to reject a writer with whom you sense a lack of compatibility. The process is supposed to be fun, not stressful. Remember that you and your ghostwriter are a team and should be able to work together as such during the duration of the project.
At the end of the day, this is your story, your project, and a reflection of you, and you want it to represent you in the best way possible. So make sure to do your homework and hire only the best match for your needs. Good luck!