Lilies propagate on their own. In fact, one bulb can have several plants shooting out from it even though it was planted just a couple years before. At first, you may enjoy this multitude of lilies, but over time, you will begin to notice the lilies are not only overcrowded, but are not growing as tall or have as many blooms as they once did. This is a sure sign that you need to separate--or divide--your lily bulbs. The ideal time to separate lily bulbs is in the fall.
Dig up your lilies. While you can use a shovel, using a garden fork will help minimize the likelihood of damaging the bulbs. Dig deep around the plants. The bulbs are probably several inches deeper than where you originally planting them. Once you have dug all around each lily, pull down on the handle of your shovel or garden fork to get the bulbs out.
Shake and brush off as much dirt as possible. You may need to wash them, which is okay. In the end, you need to be able to see your bulbs clearly to properly divide them.
Examine your lily bulbs. Use your hands to separate all the bulbs growing onto the one main bulb. You may need to twist some off, however, do not use a knife. Remove small and large ones alike.
Replant the bulbs as soon as possible or store them in a container with some moist peat moss in a cool area until you're ready to plant. You should replant lily bulbs before the ground freezes so you can enjoy their blooms the next season. If you live in an extremely cold environment, store them until the spring as soon as you can dig into the ground.