Trim at the right time. Pick up your shears at the end of lilac blooming season. Or get a head start by clipping branches of newly opened flowers to place in a vase on your table. Waiting too late in the season for trimming will not harm the tree but you will sacrifice some of next year's blooms.
Cut using the proper tools. Use pruning shears, never hedge trimmers. You want a more directed, delicate cutting effect than hedge trimmers permit.
Prune the correct way. Begin by clipping old, dead flowers at the base of the tree. Then trim away shoots near ground level. To enable sun and air circulation to reach the inner branches, trim larger stems at the center of the bush. Step back from the bush to notice if any branches protrude in an unattractive way and cut these. Most lilac lovers prefer a rounded top to the bush. According to your preference for either a taller or wider bush, encourage either spreading or lifting shoots. If your lilac tree begins to look overly bushy, trim the entire tree to within 6 to 8 inches of the ground.
If you have an older, neglected lilac tree, you can usually renew the tree with attentive care and pruning. Cut the entire plant to within 6 to 8 inches of the ground in late winter, before blooming season. This will result in a large number of shoots during spring and summer. The following winter, choose the strongest of these shoots and let them form the framework of the tree, cutting the rest away. Use the shears to discourage the selected shoots from branching. Another option is the 3-year lilac restoration plan: each year after lilac season, trim away one-third of the overall tree.