What kind of lilies? Lots of plants that have "lily" as part of their common name are not true lilies and some do need routine periodic division to regenerate vigor and flowering. If they are true lilies, you could attempt division (very carefully if done now) but in over 30 years of gardening, I've never had to divide lilies in order to encourage blooming - even very old, well-established and crowded clumps bloom beautifully in my garden. I'd look to see if something else was the problem.

I would definitely try dividing them. I moved some Liliums last spring which you aren't supposed to do. I had a large purple Trumpet lily that had had only one stem for about 8 years. When I dug it up there were many smaller bulbs. After spreading them out when I moved them I had at least 7 or 8 stems that all bloomed. Be very careful when moving them now though. If you break the stem you have no blooms this year.