A good adviser brings to the table their knowledge of the field, a suggested plan that augments the original business plan for the better, and a no nonsense approach.
They won't hold the startup's hand, but be more of a guide. A lot of startup's who aren't familiar with someone who advises (or consults) often mistake it as a means to slough off vital business and marketing lessons. They won't do it for you. They will teach you how to do if for yourself so you can rinse and repeat the situation.
An adviser is not always an investor, but someone that you can pay to consult your business. They know what it takes to get you going.
If they haven't had any experience in your niche, the best consultants out there will have a good referral list to send you. They may also have references for potential partnerships and gaining good startup capital.