Rule #1:
Love HER first, and her hair second.
Tell her (truthful) things like "If you were to cut your hair tomorrow, I'd still love you as much as I do right now."
My wife had BSL hair when we met and it's now waist length and growing. She loves to use it to excite me (and I *LOVE* it too!) and there is nothing that will stop her trying to keep it fun for both of you.
Rule #2:
She is doing it for you, so don't leave her to take care of it all by herself. Take an interest in washing, maintinaing, styling. Learn to braid. Learn to brush it for her. If you want her to keep it long don't leave the details to her alone. It must be a joint venture.
I found out early on the reason my wive never had hair as long as it is now before is that she kept rolling on it in the night. A nightly braid (which I sometimes don't feel like getting out of bed to do, but do anyway) is the price I pay to keep her from getting frustrated with her own hair and chopping it off.
Rule #3:
Let her know how much it means to you that she is silling to please you. Compliment her hair length, shine, smoothness, silkiness. Don't assume that it will always be there. "Thank you" means a lot especially in bed.
The first time I said "Your hair looks so much shiner now that it is longer," my wife actually blushed. She knew I had noticed and that I appreciated the effort she had been taking on her hair.
So far it has all been working. So we shall see if that is all there is. But I think there may be one other rule that is really all the other rules rolled into one.
#4: Don't expect anything, but appreciate everything...especially the special someone in your life.