The easiest way we've found is to dig up the clump, shake off a the soil, lay the clump on its side and gently pry off pieces using a weeding fork as illustrated below:
1. lay clump on its side, begin loosening roots
2. use your thumbs to gently pull sections apart
3. pull the sections apart
4. use trimming shears to cut off foliage about 6" above the crown
5. divisions are ready to be planted
6. Once hole is prepared, set plant at crown depth and press soil around firmly.