What advice would you give a customer looking to hire a provider in your area of work?
"Communication is king..."
Let your graphic designer know what you are looking for as clearly as you can in your initial consultation, and as drafts are sent your way don't delay in sending clear communications back about them. This will only help in keeping your design on track and on schedule.
If there is something that is absolutely not going to work for you, let them know right away so that they are not barking up the wrong tree. Once things are moving in the right direction - trust your designer to do his job for you. Give them some creative breathing room to make suggestions and additions that you may not have considered. Their creative ideas and experience will usually lead to surprisingly positive results.