This is what agencies do best. You can get too close to your product and lose touch with your customers. Keep your promotional message consistent. Recommend that the look you have been using regularly be carried through into your exhibit marketing materials. Complain when necessary, but also compliment and appreciate your agency's efforts.
Creative people thrive on recognition.
Establish a method to evaluate and measure results at the project's completion. Don't hire an agency whose business, expertise and philosophy are not in line with your business principles. Don't accept recommendations blindly. Challenge and question why the agency will work. Don't nit-pick and tell the agency how to do its job.