Apart from those perennials that resent disturbance, dividing perennials is highly beneficial to the plant but also provides you with more plant stock. A win, win situation.

Dividing perennials is simply done. As each perennial plant ages the central portion dies out so every 3 to 4 years this portion should be removed and divided with the new divisions being planted in freshly enriched soil.
Dividing Perennials should be done in Spring or Fall.
1.. Trim down the stems so that you can see the crown of the plant. Use a garden fork to carefuly lift the clump out. It is best to gently insert the fork in a few spots around the clump and gently lever loose the plant.

2.. Insert two garden forks into the clump back to back and push their handles together. For smaller clumps use a hand fork.

3.. Now using your hand gently pull the plant apart into smaller clumps. Dispose of the old woody parts.

4.. Do not let the roots of your new plants get dry before you have replanted them. Drop them in a bucket of water or cover with wet sacking until you are ready.

As you can see, dividing perennials benefits the plant and you. Keep new plants moist for first few weeks.