Breastfeefing

I had my little angel at 28 weeks weighing at 1lbs 9oz. Bless her little self she has been doing good and they keep increasing her milk dose and is now at 18 ml every 3 hrs. My problem is I can bearly keep up with her. My milk supply is low. Anyone have any tips to increase breast milk ?? I pump every 2 he's and drink plenty of H2O.