Select at least one of the missionaries our church supports and pray for their relationship with God. I'm afraid, all to often, we assume too much in this area. It doesn't occur to us that our missionaries have any need in regards to their walk with God. Pray for your missionary's physical and emotional life.
Many missionaries live and work in difficult climates and where various diseases are prevalent.
Pray for their health and safety. Also pray for deliverance from discouragement, loneliness and depression as they are in the thick of the spiritual battle. Pray for your missionary's family. Pray, if married, for the husband and wife's relationship. Pray that as a family, they will provide an excellent model for the people among where they serve.