Divide when plants look good - don't wait until they’re overgrown. Doing this will keep the plants looking their best and give them the best chance to split successfully.
Divide into quarters or more - if you split into too large of sections, you will need to divide more often. On the other hand, if you divide into smaller healthy sections (and only keep the best portions), you will split less often and have better plants.
Wait to divide until plants have finished blooming. They are less likely to have transplant shock after flowering is over.
Dig from the edge of the leaves, or ‘drip line’, which is a good indicator of the length of the roots.
Divide on a cool day and water well after replanting. The new sections will transplant better in cooler weather when the soil is warmer than the air. Also, watering the roots well after replanting will give them a much-needed boost.
Add compost to the soil when replanting. While the ground is already disturbed is a great time to replenish soil nutrients.