Most perennial plants can be divided with the simple two fork method described above. Occasionally you will encounter a perennial that has either been growing too long in one spot or that simply has a really thick root system. Grasses tend to develop such heavy root systems that just digging them out of their hole is a challenge. Trying to divide these with the two fork method almost guarantees a broken fork and a very sore back.
Densely rooted plants require cutting the root system open. This can be done with a sharp saw, by two people and a bow saw or even with an ax. These methods should all be undertaken with extreme caution. You will hurt yourself a lot more than your plant.