The best time to cut lilac blossoms is early in the morning when they're fully hydrated. Cut the stems with sharp, clean pruning shears, then immediately plunge the cut stems into a bucket of water. Indoors, get a vase ready for the flowers. Make sure it's clean by running it through a dishwasher cycle or washing it by hand with a solution of 10 percent bleach and 90 percent water. 

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Add fresh water and a floral preservative, which you can find at florist shops. Remove all leaves that would be underwater in the vase. Recut the stems at a 45-degree angle, and arrange the lilacs in the prepared container. Set the vase in indirect light and enjoy. Recut the stems, and add more water as needed to prevent wilting.