(1) If You Don't Know What You're Doing or What You Want
Ask several vendors to come in for a vendor meeting. Explain what you envision, but also explain that you're not sure what you want. Ask vendors for ideas, suggestions, and possible solutions for your problem. Most vendors are willing to take a couple of hours to help you. Make it informal, and promise nothing. Be up front, open, and honest. Take time to read trade magazines, attend conferences, and learn as much as you can about various topics related to your project.
(2) Working With the Vendor During the Project
If you promise to supply storyboards, task analysis, illustrations, technical drawings, photographs, videos, etc., deliver these materials on or ahead of time. It is very frustrating for a vendor to try and work for someone who can't or won't "team" with them on a project. If a project does fall behind, and it's your fault, don't try to blame the vendor. You may get away with it, but you and your organization's reputation will suffer in the long run. Finally, pay your invoices. Failing to do so will often break several financial agreements usually detailed in the proposal. Organizations who do not pay their bills often find their projects are set aside by vendors who must work on projects that produce a predictable cash flow in order to meet their financial obligations.