Ours only have acess to the one field as well. So what we do is rest the field every year for about 6 weeks. During those 6 weeks we have nothing on the field at all, we generally rest it from early December to mid or late January. The goats don't normally get out at all during this time, but we can take them for walks if need be. We worm fairly regularly, and get their dung sampled if we suspect any issues. We also generally keep two types of wormer (there are 3 basic types or families of wormer, a clear one, a white one, and a yellow one).
I also agree with everything Anke says above!
Beth