Prune a Japanese lilac tree twice a year, once in winter when it is dormant and once in spring after it blooms. You need pruning shears or garden clippers and a ladder.
Prune the stems
In mid-winter before new growth appears, trim about one-fourth to one-third of the largest stems back to the trunk or a main branch. Leaving only 6 to 12 major stems that do not rub each other allows the tree better ventilation.
Also in mid-winter, remove superfluous suckers, or new stems growing from the root system. Cut them right up against the trunk just below ground level to prevent them from growing into additional trunks. A Japanese lilac should have no more than one to three trunks.
Shape and beautify the tree
A Japanese lilac grows up to 30 feet high and spreads 15 to 20 feet. In spring just after the tree flowers, control its height and width by cutting the branches back to about 1 foot below the height you want the tree to be. When trimming a branch, cut it back to 1/4 inch above a bud, or swollen section of the branch or stem. You can also trim away any extraneous growth. Deadheading spent blossoms encourages additional growth the following year.