Put advisors on a short term engagement - e.g. 6 months or 12 months. It's easier to not renew a poor performing advisor than to cut one on a longer engagement. Have the advisor tell you what they can do for you and then put milestones against that. Decide how much equity you want to give. Split the equity so half vests monthly over the advisor engagement and the other half is earned by achieving milestones. That way they are getting some equity in good faith and the trust issue should not come up.
Here’s a sample Milestone Advisor Agreement: https://drive.google.com/open?id=0BxLFp1MRwjgqLVI5QWVGakNHdXNpUXpYc3RpZ3A3dnphOGRv