An Extension of the Team
Embrace consultants as an extension of the team.
Even the best consultants can quickly become discouraged if they feel like they aren't part of the team. If you aren't transparent with the consultants and don't accept them as a true extension of your team, they will never be close enough to the project to completely understand it. Both teams are working together toward the same goal, so they should be privy to the same information and treated the same.
From the beginning, educate the consultants on your team about the greater picture of your product or service, and how they play a role in your success. Communicating your vision and welcoming them to your team will create more trust and appreciation, as well as the start of a lasting relationship.