Praying for the Church’s missionaries should be a regular part of the ministry of the church. 

Print out a list of their names, maybe including additional information like the country / region where they are serving and specific prayer requests from them.

The following quote from Tommi Sitton, To Every Tribe Ministries, in Reckless Abandon: A Modern-Day Gospel Pioneer’s Exploits Among the Most Difficult to Reach Peoples, is helpful in bringing up things to pray for:

When people ask me how to pray for missionaries, I tell them they need to be remembered daily, because these kinds of stresses are everyday realities. Pray for the big things: that the gospel will advance through their efforts; that they will be sustained through loneliness; that the Lord will provide financial partnerships; and that He will protect them from illness and those who would do them harm. But also remember to pray for the everyday things: transportation to the market, strength to do tasks such as washing clothes and making meals, finding correct boundaries in relation to possessions and privacy, and enduring the stress of long periods of separation. Pray for the Lord to sustain their relationship with Him and with one another. And, particularly, pray for their children. (pp.64-65)