well, i dont think it needs to be a long detailed email unless you are going to tell her things you are unable to say in therapy, tell her things you wish you could say to her face. give her intros to things you wish you could talk about, what goals you would like to acheive by seeing her, what you think would make it easier for you to talk in session. i am sure she would appreciate knowing your heart is in it so she knows she is not getting nowhere by seeing you.