A little ballet wouldn't hurt. Or at least a ballet warmup -- particularly the tendus. These exercises are particularly good for strengthening both turnout and the arch of the foot.
When executing tendus, work the heel forward as the foot extends out, then pull the toe back as it draws in to a close (or 5th, or whatever).
If you don't have the time to take ballet on a regular basis, try just a few classes here and there, whenever you get a chance. This will at least give you the tools to create your own warmup regimen, including the tendu exercise described above.
Regards,
Jonathan