A designer must call upon other sources than themselves when presenting what they feel is the right look for their client. A committee of trusted colleagues provides you with that outside look. In my case I have friends from a wide spectrum of professions, who are more than willing to critique for the small price of coffee or lunch or a simple thank you. If the audience is the general public then a mix is needed. If the users are going to be part of a group, then get them involved. Regardless, building networks and entrusting in your friends’ talent is always a plus.