Pruning:

As a general rule, all lilac should be pruned immediately after flowering in the spring. Since lilacs set next year’s flower buds right after the current year’s flowers have faded, pruning later in the summer or fall will result in cutting off many or all of next year’s flowers.

• Deadheading – removing spent blossoms and seedheads to visually clean it up and increase the blooming potential for next year.
• Tipping: Instead of deadheading, cut back to a pair of side shoots instead of at the base of the spent flowers. Never remove more than 1/3 of the shrub when tipping. Do it right after the shrub is done blooming.
• Remove new shoots: These shoots, or suckers, are the new growth coming up from the ground around the base of the lilac. This should be done annually.
• Remove dead, damaged, diseased, or crossing branches: This is known as thinning your lilac bush. Thinning opens the bush to allow for greater airflow and helps shape the lilac and control its height. It also helps to keep the branches healthy and should be done annually, as needed.
• Rejuvenation For Older Plants:
  
  • Remove about one-third of the old canes down to the ground in the first spring.
  • The following year cut out the next 1/3 of the oldest branches then the final year cut back the last of the oldest branches.
  • Wait several years before repeating the process.
  • If you are impatient and don’t want to prune over three years, take drastic measures and cut the whole shrub down to 6-8 inches from the ground in early spring. You will not have blooms, though, for several years. Do not prune again for several years unless removing suckers or dead or diseased branches.