If you plan to cross paths with a missionary, you might prayerfully consider one or more of the following. If you will be helped by or stay with a missionary, prayerfully consider doing more:

Be thankful for their ministry and service.  Express appreciation.

Pray with them.

Commit to pray for them (only if you actually intend to).

Take them out to eat and pay for their dinner.

Prepare meals for them even in their own home.  Be sensitive that they might prefer not to have strangers searching through their kitchen and just cook themselves.
Wash the dishes.

Play with, read to, build up their children.

Leave some decent magazines from the U.S. that might appeal to women or men (current news magazine, Better Homes and Gardens, Sports Illustrated- except certain issues).

Take a video tape or DVD (find which they prefer) of their favorite TV show, documentary, or a recent sports event they enjoy (the World Series, Super Bowl, college sports, etc. don’t broadcast ubiquitously in West Africa).
Bring boxes of their favorite cereal.

Bring the ingredients for tacos (All missionaries love Mexican.  Most love Chinese.)

Ask for their opinions. Listen to their experience and insight.
Ask what God is doing in their life, in their family, what they are excited about and listen.

Don’t complain about the country in which they live, the temperature, the roads, the food, and especially not the people for whom they are giving their lives.  In fact, don’t complain about anything.  Your whole team will enjoy it.
Live in the country, do not try to replicate the U.S. out of loneliness.  Don’t beg for American food. You can eat all the hamburgers and fries you want when you get home.  Try something new.  Learn to enjoy and be grateful for God’s provision.

Look for ways to be a blessing.

Be an enthusiastic supporter of the Lottie Moon Christmas Offering for International Missions.  Tell your church how carefully and well the funds are spent for ministry.
“Do not let any unwholesome talk come out of your mouths, but only what is helpful for building others up according to their needs, that it may benefit those who listen.”

Ephesians 4:29