Rakkoon's got some good advice. There
is such a thing as low milk supply, but unless you have someone weighing the baby immediately before and immediately after a feeding, it is impossible to diagnose. PLEASE do not let anyone tell you that you have a low supply based on how much you can pump, which seems to be the usual method of "diagnosing" it. Even with my first daughter, when I was leaking copiously from one breast as she fed off the other, I could never pump more than an ounce at a time unless I was engorged. The difference between your baby's efficiency and that of the best pump in the world is massive.
I also don't buy the obesity causes low milk supply theory. Not that I'm trying to attack you; it simply doesn't make sense. If it did, thin women would be known for being better milk-producers than fat ones, and they aren't. (I'd also think that losing 35 pounds between daughters would have resulted in a marked improvement in my supply the second time around, which didn't happen.)
Using an SNS to supplement is smart of you; that way at least you're still getting the nipple stimulation. But the formula your daughter is getting will still fill her up faster and make her nurse less. A quick rundown of the usual methods for increasing supply: get a breast pump and pump between feedings (theory is this will help you supplement with your own milk), take brewer's yeast (in fact, if you can bring yourself to do so, drink a beer a day once the baby's down for the night; the alcohol will be out of your system before her next feeding if she's on about a 3 hour routine), milk thistle, mother's milk tea, fenugreek. I've also heard oatmeal helps, though I'm not familiar with that one. Make sure you nurse on demand, preferably at the first sign of hunger from your daughter. There are also two prescription medications, Reglan & Domperidone, that apparently have the side effect of increasing milk production.
As to what to do to ensure your supply doesn't drop--you have to eat enough, of course, but if you are careful you can diet gently and not affect your supply. (IMO you'd have to get down to starvation levels to affect it.) If you are staying well hydrated and eating enough protein, that
should cover it.
Rakkoon's suggestion of consulting with the LLL is a great one. You can also try consulting a lactation consultant on your own, just make sure she is IBCLC certified. The knowledge required for that certification is exhaustive, but too often the 'consultants' at the hospital aren't IBCLC certified, and my experience with them is that they only have
slightly more of a clue than the average woman on the street...You might be able to find contact info for a lactation consultant at the
IBLCE website. Also, check out
Breastfeeding.com for more info on what you can do now.