The most important advice would be this (and any advice I give is on the understanding that the person taking it has only non-sexual intentions with their boy. If someone's intentions are sexual then the only advice I have for them is "keep away from him"):

GUARD YOUR HEART.
Don't let yourself fall deeply for someone who can be snatched out of your life in a moment's notice. Work on the friendship with the whole family and get some solid ground in your connection before you get deeply attached emotionally. That can be harder than it practical... but as much as you can have control over that aspect do it. There is NOTHING worse than having a YF you love with all your heart be snatched out from under you... sigh.

Common mistake?... there I'm not so sure.
Don't be overly keen and start scaring people with your enthusiasm.
and
Don't project the things YOU need onto the boy. Get good adult support around you to look after your emotional needs so that you are a healthy person to bless your boy... rather than an emotionally needy person that is going to be overly dependant on a young person too immature to help you.

But overall.... have fun. They grow up so quick. Enjoy it while it lasts.

Blessings
Cat.