Lilacs have a tendency to wilt after cutting due to lack of hydration of the blooms and leaves from insufficient uptake of water and nutrients inside the stem.
After cutting the stems place in a container of water and set aside for several hours in a cool shady area, allowing the stems to condition and hydrate.
Cutting lilac stems involves making two vertical cuts through the stems on the bottom inch or so of stem, allowing more surface area from which the lilac stems can rehydrate.
Keep all foliage above the water line, trim regularly, change the water daily, and provide some plant food. This will also help lilacs to last longer in the vase.