About time we start this thread.
I'm all about correcting muscle balances in the individual. The over head squat test is one great way to assess imbalances, and so are push up and inverted row exams.
I think the areas most lifters neglect the most is:
Strengthening the hip abductors and the shoulder external rotators
Increasing flexibility in the shoudler internal rotators/external capsule and the hip flexors.
What stretches have you found helpful for the hip flexors besides the basic standing lunge, kneeling lunge, and lunge with knee flexion?