Answer: It sounds like your lilac is a candidate for some renewal pruning. The best time to do it is in spring right after it blooms, although earlier spring is fine, too. (If you prune in early spring you will sacrifice the blooms.) There are two methods of doing this. The drastic method is simply to cut the whole thing off at the base, as close to the ground as you can. This will result in many suckers growing up from the roots which will then be thinned every spring.
The less drastic method is to remove one third of the oldest thickest stems for three years in a row. This will also result in many suckers to be thinned, but is less traumatic to the faint hearted among us. In either case, be sure the plant receives adequate moisture and nutrients to help it recover from the pruning especially if you use the drastic approach.
Each spring after the shrub blooms, remove any suckers smaller than a pencil; also remove enough of the larger suckers and older trunks to make a well shaped shrub. Always cut as close to the ground as you can. Lilacs seem to appreciate a nice dose of fertilizer or rotted manure early in the spring, too. One other point is that some lilacs are alternate bloomers, meaning they bloom heavily in alternate years and less heavily in between, so if your lilac seems not to respond well to your efforts at first it may simply be an "off" year.
Good luck with your lilac!
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