Active range of motion exercises like writing your name in the air with your feet helps with mobility. Also, standing at the edge of a step and letting alternate heels lower down helps with dorsiflexion range of motion. Once range of motion is restored I love utilizing the fitness band for strengthening the angle in all ranges including plantar flexion, dorsi flexion, inversion and eversion which helps greatly with ankle stability.
I suggest you check out Bill Hartman and Carson Boddicker’s self mobilization techniques. I have used both after a few sprains out on the trails as well as with my clients. Carson also offers a few good drills. If you want a more in depth but fun and functional approach, Gary Gray offers a few DVD series that cover both the foot and ankle which include various ways to ‘tweak’ the range you may be lacking. Good luck.
Nick, it is great that you have acknowledged that you have little knowledge in this area. You might want to first increase your knowledge of the ankle joint.
Once you know the the mechanics (i.e. the names and the shapes of the bones, the muscles that surround the joint) you are in a better position to serve your clients.
Hope this helps.