Here an adjustment you can try to make:
1. Open your trunk up
2. Look underneath the lid.
3. On either side, you'll see a couple of what look like rubber sockets. These come into contact with the metal just above the tail light housings. They're threaded -- and therefore adjustable. Turning them clockwise should shorten them up -- allowing the trunk lid to close more firmly against the rubber seals.
NOTE: Be sure you don't make the stops too short -- otherwise, the bottom of the trunk lid will bang into the top of your bumper. Another problem is that the trunk lid might gently rest on it -- which will scratch the hell out of the bumper cover. Put a good coat of wax on this area first. When everything's perfect, the trunk should pop up a little when you hit the release, not leak, and close easily without slamming, etc.