I came upon an article from Fine Gardening that is quite extensive on tips for dividing perennials. Since my husband and I focus on hostas and daylilies we often divide them. We sell plants at the annual BSHDS Plant Sale in Perrysburg, OH each May and we donated plants to Simpson Garden Park in Bowling Green, OH.
Janet Macunovich is very thorough in these tips, includes photos, and provides video as well.
Click on the button to reach the link. Enjoy!