While lilacs can live for decades, often not being considered old until they hit 100 years, many lilacs will slow blooming or produce smaller, fewer blooms because of lack of care. To get a lilac bush to bloom large, healthy flowers year after year, you'll need to care for your plant lightly each year. The lilac produces blooms on old wood, so you never want to cut all the branches back, hoping to increase production. But careful trimming and a little fertilizer can go a long way.
Rake around the base of the lilac bush to pull away old mulch, dropped leaves and dead branches. Discard this material or place it in a composter.
Apply a general purpose fertilizer in early spring according to manufacturer's instructions when you first notice the buds of the lilac starting to swell near the tips of last year's branches.
Replace the old mulch with fresh mulch 3 inches deep around the base of the plant.
Look for a few, weak blooms to emerge and begin to die off and dry. The dry blooms are your signal that it is time to prune. If your lilac experiences no blooms at all, allow four weeks to pass after step 2 before moving ahead to step 6.
Snip each spent bloom off the bush with your hand pruners by cutting just behind the bloom. At this point the lilac is already working on the next year's bloom, so only cut behind the bloom and not further down on the limb.
Look over the lilac to spot old and damaged branches. Follow the old branches down to their base and cut them off horizontally about an inch from the ground. Leave four to five strong branches behind no matter how many old branches are removed.
Fertilize the lilac once more following flowering and trimming. Allow the plant to grow undisturbed until the following year before repeating fertilization annually before and after flowering. Trim according to step 5 annually, but perform step 6 only every three to five years.