Hi T. -
My daughter was a preemie as well, 6 weeks. While in the NICU, they did give her formula occasionally while I was still trying to get my supply up. I was ok with it b/c I knew she needed calories. I always tried to breastfeed, but she was just so little to latch. I pumped, a lot. Once she was home, we still fed her bottles of breastmilk b/c she still didn't know how to latch yet. During that time, I was also seeing the LCs at Kaiser.
Off and on, I used a nipple shield, and that helped a lot. It helped her to get more milk out of me while nursing, without becoming too frustrated and expending too many calories while trying to nurse. In the end, it took her 9 weeks to learn how to latch, although she still got a bottle of breastmilk once/day or once every 2 days. I was going back to work soon so she needed to get used to the bottle regardless. She's 6.5 months now, still nursing.
While I didn't have a low supply issue, I wrote the above to suggest maybe using a nipple shield. Some people don't advocate it, but it helped us a lot. Also, it can be really frustrating, I know you are trying. Don't beat yourself up if it doesn't work out.
A good resource re: increasing milk supply: http://www.kellymom.com/bf/supply/low-supply.html
Good luck,
H.