Arapahoe County Colorado
Not all blooming shrubs are alike, so it is best to understand how they bloom. Pruning and thinning at the correct time will affect the next season’s flowers.
Lilacs are known as spring-flowering shrubs that bloom on one-year-old wood (twigs that grew new the previous summer). Buds develop mid-summer through fall for the following spring. Spring flowering shrubs can be pruned by thinning in early spring before growth starts or immediately after they are finished blooming.
After blooming, use long handled bypass loppers or a pruning saw to remove 1/3 of the oldest stems and those that are more than 2" in diameter. You can also remove malformed stems, twiggy suckers, and any dead or diseased stems. These should all be removed as close to the ground as you can possibly get. Next year, remove another 1/3 of the oldest stems, and the third year prune out 1/3 of the oldest and so on.
Here are a couple of links that might help you understand your flower shrubs and when to prune them:
The University of Maine has a very good detailed how-to video: