Hi, You will want to prune your lilac bushes back when they are finished blooming and the blooms start to die back if you are trying to maintain size. If you prune too early in the spring you will remove the flower buds. If you are thinning you will want to thin out the thicker older canes, removing them to the ground. The younger canes will produce better flowers. This can be done to keep the plant rejuvenated or to restore an older lilac. Canes can be removed in late winter/early spring.