Asiatic lilies are hardy lilies that are easy to grow, even in colder zones. They have large exotic blooms with a sweet smell and come in several varieties including "Giraffe" and "Karen North." While each Asiatic variety is unique with different style blooms and stems, they should all be pruned in a similar manner.
Prune the flowers after they have bloomed. Simply snap the old blooms off with your fingers.
Wait for the foliage to turn brown before pruning. It may be difficult to wait since after blooming, Asiatic lilies may not be considered attractive. However, depending on the variety and style of the stems, you may be able to bundle the leaves in your hand, fold them over and tie them together with some string. This will still allow the leaves to absorb sunlight and turn it into sugar through the process of photosynthesis, but not take up as much room in your garden. You can then plant blooming annuals around these bundles to bring color back to your garden, if you desire.
Use clippers to cut off the foliage, about an inch above the soil. For healthier lilies, do this only after the leaves have turned brown, generally sometime in the fall. You can wait to prune them back, but be sure to get it done before the Asiatic lilies start growing again in the spring.