Well - for starters, ISTJ females like consistency, so don't become overly emotional or overreact to situations that aren't that important. At the same time, we look for support.. and since we generally put a lot of thought and fact-finding into our decisions, we usually make fairly good choices on a responsibility level. However, what I think we look for (or at least I do) in a potential mate is someone who is supportive, friendly, consistent and of course more outwardly emotional than myself... I need someone to bring me out of my shell!
Don't go overboard and tell her all of the dreams, desires and fantasies you've ever had about your relationship... but let her know that you feel lucky to have her and that you're happy with your relationship.
I guess unless there are more specific questions, that is my personal opinion on the matter