Here's what TOH
landscape contractor Roger Cook had to say when reader Bev Nelson in Napoleon, Idaho, asked for the best way to prune lilac bushes so they'd bloom every year:
"Several things can cause lilacs to bloom poorly. An unusually cold winter or late cold spell could have killed your lilacs' flower buds, one of the shrub's more vulnerable parts. Or, your soil might have become too alkaline or acidic. Lilacs like soils whose neutral pH is between 6 and 7. A soil test will answer this question. Also, you should not overfertilize lilacs; that will cause them to produce more leaves than flower buds.
But the biggest reason lilacs don't flower is because of pruning at the wrong time of the year. Lilacs bloom early and set new flower buds early, so a pruning in June or later in the season takes away the buds for the following spring. The best time to prune is right after the flowers turn brown.
On large lilacs that haven't been pruned for a while, I'd prune one-third of the largest branches the first year, one-third the second year, and the remaining third of the older branches in the third year. Blooms form on stems at least three years old, so you'll never be without flowers. And the result will be a totally rejuvenated plant, with larger, stronger branches that produce more flowers."
For more of Roger's pro advice, see Pruning Lilac Bushes