I would suggest checking with a lactation consultant at your local hospital for further input. Or connecting with a Nursing Moms group. They can be incredibly helpful. For me, the first couple of months of nursing were the hardest. I chuckle, thinking about my husband and I arguing about the right way to do it, worrying our son wasn't getting enough nourishment.
Do you have a double or single breast pump? If just the single, it might help to get a double one like the Medula Pump in Style. I'm pretty sure they still make those and sometimes health insurance will kick in a little of the cost. Ask to see what the insurance company says. It couldn't hurt to mention that she is having a hard time latching on. Insurance companies may listen more with that in mind.
And drink lots and lots of fluids, trying some of the dietary suggestions already listed below. I found that relaxing music helped immensely and getting enough sleep made a difference as well. Plus, try having a favorite picture of your daughter or even tape her crying or making sweet sounds if she can't be there while you are pumping. Having your child trigger your flow could potentially help with your milk production. Also, warm cloths on your breasts may help the flow as well.
If you ever get her to latch on successfully, make it as pleasurable an experience as possible for both of you. If you are stressed, your milk will not flow as well. It can become a vicious cycle.
I used to nurse in a big old leather rocking chair with a pillow on my lap and our son on top of the pillow, snuggled in my arms. I would make "letter tapes" to him, talking to a tape recorder about various new things he was doing or what was going on in our lives at that point in time. (I still have them for him to listen to. Our 12-year-old likes to listen to bits of them every once in a while.) They include me talking but also him making his little baby sounds. If your daughter never latches on, you could do it while you are pumping to help get your mind off of the pumping. (Of course, you'll also have the sound of the pump. Nothing like the sound of a milking machine to draw a chuckle or two!)
Generally, the more you pump, the more you will produce. Breasts generally respond to supply and demand. But it takes some diligence, even when you feel like you're not pumping much. As for latching on, I know it's frustrating but if you can keep trying to have her latch on, that might be the final trick. The sucking of a baby is MUCH stronger than any pump can replicate, and that's the best trick for getting your flow up.
The problem with supplementing with formula is that it fills your child faster than breast milk. Plus, when they are at their hungriest is when they suckle the strongest, thus, getting your breasts flowing the most. So, supplementing can be counterproductive to your flow.
However, not everyone can breast feed for various reasons. We have a friend who's daughter never was able to get enough nourishment from breast milk alone, so supplementing was the only way to go. She went from almost listless to healthy with the switch to formula.
I hope everyone's suggestions give you some solutions. Best of luck!