1. Divide when a plant looks good
Don’t wait until a plant has become decrepit or monstrous to divide it. My rule of thumb is when it looks its best, divide it at the end of that year. Watch for the early signs of trouble: when the center of the plant has smaller leaves, fewer flowers, and weaker blooming stalks than the outer edges like this Sedum
‘Autumn Joy’, or when the plant runs out of growing room on its edges and has nowhere to go but into neighboring plants.