Mosiah 27

14 And again, the angel said: Behold, the Lord hath heard the prayers of his people, and also the prayers of his servant, Alma, who is thy father; for he has prayed with much faith concerning thee that thou mightest be brought to the knowledge of the truth; therefore, for this purpose have I come to convince thee of the power and authority of God, that the prayers of his servants might be answered according to their faith.

There is great power in prayer. I haven't always exercised enough faith to continue in prayer on behalf of those I love to ask the Lord to bless them and help them. He will hear our prayers and bless those we pray for. I have felt the sustaining power of the prayers of others. President Monson asked us to pray for him and for those who serve with him. The epistle of James contains the following instruction (James 5):
 
15 And the prayer of faith shall save the sick, and the Lord shall raise him up; and if he have committed sins, they shall be forgiven him.
16 Confess your faults one to another, and pray one for another, that ye may be healed. The effectual fervent prayer of a righteous man availeth much.