These lilacs have gotten very large over the 3 years I've been at this house, and the most I have done with them is give them a good shearing after they bloom.
As you can see from the pictures, 2006 it was sporting a flat top and was very meatball shaped (must have had the grandpa over to help shear);
by 2007 it came into it's own with a beautiful naturalized shape,
and then 2008 was bloom-rific if not getting pretty big. And it doesn't look like it needed pruning but the inside was just all wood.
So after renewal pruning, it now looks like this:
Kinda small, kinda sparse, kinda sad. We'll see how it takes for next year.