Here's a short but very interesting article:
Serving Others Contributes to Spiritual Growth
by Eric Swanson (Christianity Today)
I surveyed my church to see if people saw a relationship between ministering to others and spiritual growth.
When asked, "To what extent has your ministry or service to others affected your spiritual growth?," 92 percent answered positively. None responded that ministry had a negative effect on their spiritual growth.
Sixty-three percent indicated that service was equally significant in their spiritual growth compared to other spiritual disciplines, such as Bible study and prayer.
Twenty-four percent responded that ministry or service to others had been "a more significant factor" to their spiritual growth than Bible study or prayer.
Over half (58 percent) of those who were not actively ministering to others felt either "not satisfied" or "somewhat satisfied" with their level of spiritual growth.
—Eric Swanson, What You Get from Giving (Leadership Journal, Spring 2003), p. 37