The Soul of the Apostolate is a book of which I have been vaguely aware for many years, but I never had the discipline or inclination to read it through until very recently. The premise is as simple as the Gospel: "Without God, you can do nothing."
St. Augustine, I believe, said, "Pray as though everything depends on God; work as though everything depends on you." The reality is both: Everything depends on the action of grace... and we can't shirk our duty either.
In pondering a vocation (or in helping someone to discover a vocation), prayer is the absolute prerequisite for anything resembling success. And not just a prayer here or there - it must be an active seeking of the will of God in daily, intense, prolonged prayer.
More on the "how to" later.