Prune all but one, two or three main stems of a newly planted lilac. Prune all the stems off at ground level using a pair of garden clippers or lopping shears.
Cut off the bottom side shoots growing from the main stems to form a trunk. Cut them right next to the trunk with a pair of garden clippers. Trunk heights can vary among gardeners, but for a young lilac, a 12- to 18-inch trunk should suffice.
Remove suckers or new shoots that grow from the ground whenever they grow. This will keep a lilac looking like a tree rather than a shrub.
Prune a lilac tree once every three or four years after flowering as desired. Trees grown as lilacs can continue to grow without pruning, and only the suckers need to be removed. However, to control growth or to encourage a fuller canopy, prune up to one-third of the stems every few years. Cut stems that face outward and clip them about 1/4 inch from the parent branches, making sharp cuts with a pair of hand clippers. Choose the oldest, least productive stems to prune.