When is the right time to cut back lilacs? 

The blooms on ours are gone for this year (always so brief!!)...and I'd like to cut back some of the straggely branches and "reshape".  Is now the right time?

Thanks for any advice!

My undertanding is that, since the plants bloom on new growth,you want to prune right after they have quit blooming.  That way, they will put out the new growth during the remainder of the summer, and will bear flowers the next spring.  If you wait until later (like now), you will be cutting off the blooms of next spring. 

David, thanks so much for replying!  I will know the right thing to do next year, and that's a big help.  Thanks!