Well, to start with, just because you do not feel your breasts full of milk anymore, does not mean you do not have enough milk! About 4 months after birth your body learns to regulate supply with the demand. I nursed till 24 months with 2 of my kids and after about 3 months I never felt my breasts full, but they supplied plenty of milk, exactly the amount my baby needed.
Secondly, supply = demand, so if you give her formula, she will not be nursing as much, which in turn will cause you to reduce milk production.
Thirdly, get enough rest, sleep together, nurse on demand ( do not schedule your nurses), drink enough and eat enough protein. Enjoy some Raspberry Leaf Tea too ( it is real nice for nursing)!
I think you will find the best and the most comprehensive information here:
http://kellymom.com/bf/got-milk/supply-worries/low-supply/ This is the best compilation of resources on everything breastfeeding and I highly recommend it!
Wishing you all the best, you are doing great!