James encouraged a mutual accountability among believers in prayer. This level of fellowship is rare in the church today, but is much needed. Healing in body, soul and relationships is experienced when believers are honest about their needs, weaknesses, and sins in prayers together.
Do you have one or two close friends of the same gender that you share mutual accountability and prayers with? Do you meet regularly? Gathering together regularly and authentically is “effective.” And doing so fervently “avails much.”