To keep your lilacs looking their best, they need to be pruned, fertilized and shaped almost every year, soon after they are done blooming in the late spring. Lilacs generally produce more blooms on healthy, newer growth, rather than on older stems, according to Ross Penhallegon, horticulturist with the OSU Extension Service. Don't wait too long, as next year's blooms start forming in the summer.
If your lilac is very thin and lanky, renovate it by removing the oldest branches gradually over two or three consecutive seasons. The first spring, remove about one-third of the old canes all the way down to the ground. If your lilac is a grafted variety, make sure you don't prune below the graft union.
Over the next couple of years, take out the remaining two-thirds of the old stems. Your lilac bush should respond and fill in with lush, healthy new growth from the base, with more flowers on the lower stems.
Once you have your lilac looking good, Penhallegon recommends that each year you clip off the faded flowers to stimulate new growth and more blooms next year. Fertilize with a good general organic or conventional fertilizer in spring and keep overhead trees thinned to allow ample sunlight to stimulate lilac blooms.