Trees, Shrubs and Conifers: Revitalizing a lilac, 2 by JJeffH

JJeffH wrote:
In zone 5a, Vermont, I have an overgrown lilac that has all but stopped flowering. It sits at the corner of our porch and provides good privacy, but we want to reduce it from about 10.5’ tall to 8.5’ or 9’ tall, and trim it back significantly on the sides. We want to get it to the right size, then maintain it there with modest pruning once a year.
I have trimmed it each year for the last 3 or 4 years but I’m clearly doing it wrong because the blossoming problem has gotten worse and every year everything I trimmed grows back. I’ve heard of the 1/3 a year for 3 years remedy, but I’m afraid to be too aggressive. I’m not sure how to identify the third to be removed in any given year.
This year, just this week, I did get inside and pull out two large trunks (1\" to 1.5” diameter) from the center. That opened up a lot of light and air from above. Then I started trimming (but stopped when I discovered a robin’s egg in a nest).
I’m asking for any general advice, but I especially want to know how to tell useless suckers that should be cut out from healthy new growth in the center of the bush. I also want to know how I might be able to revitalize the soil. I do have a good compost pile and I will be spreading that soon, but perhaps I need some lilac friendly fertilizer.

Thanks,
- Jeff