I know some professional lilac nursery folks who have indicated that you don't have to dead head. Certainly there are old abandoned farmstead ones that no one dead heads and they grow and bloom just fine. I've experimented the past couple years not dead heading and it didn't make much difference with the blooms the following year. I may still do some for aesthetics and I am usually cutting blooms to bring inside as it is.

i was diligent last spring about deadheading the lilac in my front yard, and i am sad to say that i had a lot fewer blooms this year than last. the lilac in my backyard that i did not deadhead at all was chock full of blooms this year...go figure!!! my lilac deadheading days are over!

To make lilacs last longer get those packets of the mix you get with cut flowers at the grocerers.They sell them her for 10 cents a pack lilacs last a few more days with it.