Just as Bionic says. In fact, it's a good idea to pick off the worst veg matter (VM) before the sheep is shorn if it's yours, and to dag it. Don't go for a dirty or heavily contaminated sheep when choosing which one's fleece you will use. Once shorn, you lay out the whole fleece and pick out the VM by hand, first one side, then turn the whole thing over and do the other side. This also tends to allow dried mud etc to fall out.
There are various questions on here in the crafts section about how to wash fleece, so it's worth searching through. Basically, handle gently, doing the opposite you would do for felting - no shocks.