Lilacs should be pruned after they blossom in the spring, nice because you get to enjoy their spring flower. You can remove up to 1/3 of the stems at one time, so it may take you a few years to correct them completely. Most pruning guides tell you to remove the older stems, but I prefer mine to look more like small trees, so I cut them back to 3-5 main stems - like the ones on Mackinac Island