Unwrap and wash the stems, and then trim the stems ends at a 45 degree angle to help water absorption. Get rid of any foliage that will end up below the water line of your vase; this will only divert valuable hydration away from the stems.
Use a clean vase or container with fresh water, and change the water every few days. A typical head of freesia will have five or six flowers - remove individual wilting flowers to prolong the overall lifespan of your bouquet.
As with all flowers, avoid drafts, heat sources and direct sunlight as much as possible. Freesia enjoy relatively cool temperatures.