The success formula for the miraculous accomplishments of Ammon and his brethren is given here. The same formula will work for each of us: repent, exercise faith, work hard, pray continually without ceasing; then great knowledge and understanding of the mysteries of godliness will result (D&C 76:5–8), and through our mortal missions we can see thousands of souls begin the same process of repentance and return to God.
And if we do all of the above and do not see thousands of souls returning to God—does that mean that we have failed, that we were bad representatives of the Lord Jesus Christ? No. We are actually in good company. Noah spent many years preaching the gospel and crying repentance (Moses 8:23–27), but relatively few accepted his message. Lehi spent years preaching the gospel and crying repentance (1 Nephi 1:18–20), but relatively few accepted him, either. So it was with other ancient prophets and missionaries and with modern prophets and missionaries. President Heber J. Grant, for example, had no measurable success in Japan a hundred years ago. Many serve faithfully in numerous countries throughout the earth and experience little success by way of convert baptisms. But if we can kneel down every night and sincerely say to Heavenly Father that we have done the best we could, we are good servants of the Lord.