Hi William,
I agree with Drewhound. Lilacs should be pruned within 2 weeks of finish of bloom and the spent flower stalks can be removed. They set their buds early. If you need to reduce the size of your lilac, prune the oldest stems flush to the ground. Note that new sprouts usually take up to 5 years to bloom. Take a look here.
http://www.gardenersnet.com/lilac/lilac02.htmhttp://www.ipm.iastate.edu/ipm/hortnews/1993/2-10-1993/lilac .html
Newt