Shrub Trimming Basics
As a rule all flowering shrubs are best trimmed right after they bloom. Evergreen bloomers like azaleas and rhododendrons get trimmed in late Spring. Summer bloomers like Rose of Sharon can be trimmed in late Fall or early Spring. Evergreens like boxwood and cypress are best shaped in early Spring. If they are fast growers you may also trim again in Summer.
If you enjoy the berries on your holly then trim in Spring to avoid interfering with berry production. With Lilacs cut the largest stems to the ground every couple years. This is important to control the bugs that live in the wood. Basic lilac trimming is like all bloomers, right after blooming.