I had read that to get lilacs to hold after cutting the stems should be scraped and split and the placed in hot water. I tried this treatment for the past couple years and found that the lilacs were usually wilting within a day.
This year I returned to where I had read this advice in the first place - Carolyne Roehm's A Passion for Flowers. It turned out I had not read the fine print carefully enough. Miss Roehm specifies the water should be between 120 and 150 degrees Fahrenheit, and I had used hot water from the tap which turned out to be only 110 degrees. I decided to split the difference and place the lilac stems in water that was heated to 135F. I left them there a few hours until the water had cooled. Be warned that this water is hot enough to brown any actual lilac flowers that get in the water, so make sure only the stems are placed in the hot water.
The additional 25 degrees appears to make a difference because the lilacs seem to be holding better this year, ususally 4-5 days. For my customers this is unfortunately only 3-4 days because the flowers are conditioned a day before they are delivered, but the results are encouraging.