Prune out one-third of the oldest stems at ground level in late winter or early spring. Do not do this if the lilac is newly transplanted or less than 5 years old and does not have an undesirable spindly or otherwise unattractive appearance that calls for the more extreme rejuvenation pruning.
Remove dead flowers as soon as they turn brown and shrivel in late spring or early summer to improve the plant's appearance. Cut off flowering stems as close to a herbaceous joint as possible to minimize unsightly, bare stems.
Prune the lilac immediately following bloom. This type of pruning should occur annually. If rejuvenation pruning is not also being performed, remove a few of the oldest stems at ground level. It is ideal to maintain a clump of about 20 stems of varying ages.
Thin out vigorous top growth immediately following bloom. This will allow more sunlight to reach the center of the plant and increase flowering.
Prune out any diseased, damaged, especially vigorous, out-of-place or pest-infested branches or stems as they appear, regardless of the time of year.
Remove one-half of the remaining old stems, as well as less-vigorous new shoots in late winter or early spring of the second year if rejuvenation pruning is being performed.
Perform the annual pruning to thin out vigorous top growth immediately following bloom. This is when the majority of shaping to maintain ideal form should occur.
Remove any old stems still present in the third spring pruning if a rejuvenation pruning is being performed.