The best way to combat resonance in a subwoofer enclosure is with internal bracing. But that the job of the designer/builder. The consumer cannot add internal bracing, as the braces eat up some of the enclosure's internal volume, which affects the sub's performance characteristics.
For a view of great internal bracing, head over the website of Salk Sound. Jim installs the Rythmik servo-feedback subwoofer system in enclosures of his own design and build, and his bracing is the best I've ever seen. Check out the illustration of the interior of the Salk/Rythmik 12 sub!