Trim back a new lilac bush when it is 6 to 8 feet tall. Prune older bushes when the flowers fade so that the next year's batch of flowers can start to grow.
Lilac bushes grow best when they contain a combination of new and aged stems. The fresh stems do not sprout flowers for several years, so it is advisable to trim the bush regularly so that the flowers on the older stems continue to grow.
To protect the developing stems and simultaneously allow the older stems to bloom, carefully trim a maximum of one-third of the stems per shrub annually. Maintain eight to 12 stems at different stages of development, each measuring 1 to 2 inches in diameter.
Start by removing dead and slim stems. Also cut away stems that are greater than 2 inches in diameter and those that inter-cross one another or are overgrown. To prune properly, slice back and keep three to four buds on each stem. The stems should measure 5 inches in height following trimming. After tending to the older stems, trim the newer stems so that the buds face outward and fill out the bush's shape.
If the lilac bush becomes too overgrown, sacrifice the thickest stems for a period of three years until the new stems comprise the majority of the plant. It is also possible to chop away all of the old stems until the bush is 6 to 8 feet tall, and then tend to the newer stems while reshaping the plant. When the buds begin to branch, maintain the lilac bush through regular pruning.