Thanks for writing. Without being able to see your lilac in person, it's tough to say what the best way to do this is. If it's a dense, thick shrub, you might prune out a couple of the oldest, thickest stems this year. That will allow new shoots to grow; then next year you can prune out a couple of the oldest, thickest ones again. Doing so will help keep the plant under control without drastically reducing the amount of shade it produces.
---Justin, Senior Garden Editor, BHG.com