Earlier this month, I wrote this post about implementing the advice you ask for. I assumed that you had A) realized you needed help and B) asked somebody you trusted to give it to you. For some reason I've decided to work backward, because this post is about deciding whom to approach for wise counsel.
Essentially, it comes down to trust (as with a lot of things). Do you trust this adviser as a person? It helps to share basic values, otherwise either you'll be less likely to use their advice or you won't think it good in the first place. The mentor I approached about marketing was a longstanding friend. I knew him well enough to be sure that I'd be comfortable with the reasoning behind his suggestions.
Second, do you trust this adviser in the field you're asking about? Besides being someone you respect, they be knowledgeable about some aspect of your business. One time I spoke with a friend about Keyboard Confidential while the project was still in its nascent stages, and his negative reaction to my pricing affected the way I viewed the series for a long time. Be careful whom you to talk about projects when they're early and you're not yet confident about them.
Aside from trusting the adviser as a person and expert, it can also be helpful if they're in businesses that are complementary to yours, meaning that they have a similar clientele but offer a different service. With Keyboard Confidential serving the type of customer who has access to a grand piano and space large enough to seat several people, fundraising consultants and visual art advisers can help give insight into my target market that a laundromat owner might not.
So when you consider whom to seek advice from, think about whether you trust them as people and as experts. They could be friends, friends of friends, authors of books, or even bloggers. Also, identify people who work with similar customers. You'll end up getting more good advice than you know what to do with. Seriously.