Like many plants Liliums or Lilies will need to be divided after a few years in the ground. As clumps grow they become crowded, the flowers become less abundant and the stems thinner and weaker.
In extreme cases the whole clump will just collapse and rot, so divid and multiply.
Before you start you might want to establish where you want to replant the bulbs. You will need more space if you are going to keep them all.
Prepare the new location by diving in some well rotted compost and manure, dig out around 6 inches of soil so you have the right planting depth ready.
Chances are that when you dig the clump it break into a number of pieces naturally, however you should separate each bulb. Try not to break of the roots as you do this.
Gently remove the soil and discard any diseased or damaged bulbs.
You could wash them, however, generally not needed.
You are now ready to replant.
If you have already prepared a new area as suggest simply place the bulbs around 6 inches apart. Backfill with soil you have removed and water in with a little liquid seaweed fertiliser which will stimulate root growth.