I don't cover up. I do
bring a blanket along with me when I'm out, but only use it to tuck underneath DS so that a) my horrid stomache doesn't show, and b) the blanket will catch any milky drool so that my pants or skirt don't get soaked.
I've covered up a few times while nursing in front of my grandparents (but stopped after they made it clear that they were "hip" and it didn't bother them), and I used a blanket to cover up for the first few weeks while out and about with my first baby. But once I got the hang of latching her on, I abandoned the practice of hiding what I was doing. My babies have always been very good at keeping me discreet, anyway. They both drape their arm over the top of my breast, so absolutly nothing shows.
I've never worn a nursing shirt, but will probably look into getting a nursing sweater of some kind as mentioned above. We'll be spending a lot of time outdoors this fall, and I don't want to get chilly while nursing!