If you can, handle them gently. They'll love your stuff, they'll love you, and they'll make mistakes. See them as human, and treat them gently, and they will remain your staunchest allies.
Lawyers come from a different culture from most fans.
They know that standard procedure in the legal world is to send out a brash and threatening 'Bigfoot' letter. This just hurts your fans, and drives them off. Just ask nicely first. Don't even mention lawyers in the first letter if you can avoid it. The fact that you, the creator of their favorite thing, has 1. noticed them and 2. taken the time to ask them nicely will go an amazingly long way.