Strength is on the mind of most martial artists when it comes to supplemental training, followed closely by flexibility. Unfortunately you can develop both of those attributes and still not have the mobility to perform well. There are a number of ways to develop mobility, and most of them work, but you need to find one that will keep you interested long enough to get benefit from it. I'm a huge fan of using Indian Clubs to develop upper body mobility and I also use the Foam Roller every day to loosen up my legs, back and chest.
Another method would be to study Gimnastica Natural or even Systema. They both have excellent drills for developing holistic mobility and body awareness.
