In the beginning, I used Bravado! nursing bras. They were excellent.
After my boobs deflated (does that sound weird?) I switched to Glamourmom nursing tanks. I have 3, and they were and are pretty much a staple under anything I wear.
Next I went back and forth between the nursing tops and a low-cut v-neck top with a regular (non-nursing) bra. Hanes makes a great no-underwire bra with removable straps that I've used as a nursing bra.
Now that my son has figured out that he can pull down any top and expose me just about anywhere we happen to be, and also picks at my birth marks on my chest, I wear a cami, with or without bra, under a high-neck sweater. The cami does not have a built-in shelf because I don't like the pressure on my ribs -- reminds me too much of being pregnant. I guess I can get away with it since I'm a B-cup on a good day.