I have a few old lilacs, definitely decades old, and definitely neglected and overgrown. What have the rest of you done to reinvigorate old lilacs? And what time of year do you do whatever it is you do?
Work like you were living in the early days of a better nation. --Oysterband
After they bloom this year, cut 1/3rd to the base, next year (or 2 if if doesn't recover strongly) second 1/3rd, third year final 1/3rd. That should rejuvenate your lilac. Give it some compost tea if you have any available, or normal compost/fertilizer.
Diego Footer on Permaculture Based Homesteads - from the Eat Your Dirt Summit