The balance resides inside us; we have to decide ourselves, how we have to achieve that balance in our lives by keeping our souls pure through good deeds.
How you achieve balance and self-development - by training the soul, the body, or both.
Should be both i.e. soul & body. Reason being developing the soul strengthen our inner ability & developing / training the body enhanced our external or physical capability. Both work hand-in-hand to achieve maximum results. Think we need to do a self-searching or seek external advice to find out where is the imbalance so that we can do the appropriate right-mix.
For me, everything moderate is good for people - moderate sports, healthy eating. The training of the soul can be expressed in kindness and care for others, lack of envy, forgiveness, observance of religious canons etc.
Actually both different in many ways still they controls each other if soul imbalance then body will affects, if body imbalance then functions of soul will affects yoga gives training for both
I am in agreement with many of the other answers but we must not forget the importance of the role of sound ethics and also Will in our spiritual development.
Balance of the mind & the development rest with the individual as in which line he remain alert for the progressive traits which its introspection for carrying out the action of his life .
For achieving the Balance of mind we require calm & quite under the serenity where we may take the recourse to power of prayer, Meditation & such related areas for our silent observation of our mind .
mind is trained by thoughts because its upsets by thoughts only an the raining of the body through proper actions because it upsets improper actions or laziness.