Pruning Ornamental Trees and Shrubs
Purdue Cooperative Extension's Pruning Ornamental Trees and Shrubs is a 12-page online booklet that explains the hows and whys of pruning, then describes the methods for pruning different types of trees and shrubs in great detail. The illustrations that accompany the descriptions are particularly useful. The Web page is in PDF format, so you can print it out and bring it right into the garden with you.
Clever Gardening Technique
If your lilac has stopped flowering well, it might need some rejuvenation. Stems thicker than 3 inches do not flower as well as stems 1 to 2 inches thick. So cut one-third of the thick stems to the ground every year for three years. If you do this now, you might sacrifice some flowers, but it's easier to do it before the leaves obscure the stems. Rampant sucker growth also can impede flowering, so removing some of them every year is a good practice. When you cut flowers, make the cuts just above a node to promote side shoots, which means more flowers next year.