I just thinned mine out today, while it is in full bloom!  Giving it a little trim now shows off those lovely purple flowers that sometimes get lost under the leaves.  The picture below shows the effects of a little trim.  The right side shows before the trim and the left shows after the trim. My Lilac now looks like it spent the morning getting fluffed and buffed at the spa!  Subscribe to get a free video tutorial to see how I gave this lovely shrub its annual beauty treatment.