There are tons of things you can do. Just make sure your ends are healthy so you can retain growth. If you see them starting to split, break or looking a bit damaged you can trim them with professional hair shears (hair dressers sometimes get a little scissor happy).
Also make sure that it is moisturized, keep up with your deep treatments.
Make sure you are staying hydrated and getting proper nutrition.
Don't flat iron or use any chemicals including dye.
Be gentile with your hair and use a satin pillow case so you protect it while you sleep.
My hair reaches the top of my butt right now when it is straight, what helped me the most was cutting out the heat styling and keeping my hair moisturized.
You could also try scalp massages to help stimulate blood flow to your follicles.
There are tons of other things you can do to take care of your hair, healthy hair will grow and retain length. I'm sure you can find more info on this site in the articles section.