Get involved with a missionary even before they go to the field. Learn about their pre-field preparation process. Help them and encourage them with that.
Bring a friend along with you as a sender. Become a mentor and trainer for other senders.
Read the material on "best practices for missionary care" (or equivalent).
Get Propempo to do training at your church on sending and missionary preparation and care.
Read biographies of missionaries with a view to learning how they feel and what their needs are throughout their career.
Just ask your missionaries what makes a difference for them and what you can do better.