Prune lilac bushes in summer, right after they're done flowering, so they have enough time to develop flower buds for next year. Don't prune them in the spring before blooming or you'll trim off developing flower buds.
If they are the tall old-fashioned lilacs (6 feet plus), as a rule of thumb, remove one-third of the old branches each year, cutting the selected branch just an inch or two above the ground.If the bushes are seriously overgrown, you can cut the entire bush back to just a few inches tall, but it will take two or three years for the bush to recover.The most common reason for lack of flowering is too much shade. Lilacs need full sun, that is, 6 to 8 hours each day of unfiltered, direct sun. If the bushes are new or small enough, consider moving them to a sunnier site.