SUPPORTING MISSIONARIES ON THE FIELD:
First and foremost, pastors should be aware of the dangers that face their missionaries and be prepared to assist in an emergency situation. Some organizations that offer support in crises are listed on the Crisis Management page.
Missionaries are not super-human or super-Christian. They are normal people who struggle with all the things that others on your staff and in your congregation struggle with, but with the added stress of cross-cultural living and the added difficulty of isolation. They usually have few in their immediate circle with whom they can share their struggles, and they are frequently forgotten about by those they left behind. Be aware of this and keep in touch with them. Provide them with the same support and accountability as you would anyone on your staff or in your congregation. Provide them with opportunities to go to missions retreats and conferences. There is an extensive list of counseling and retreat centers specifically for missionaries on the pages linked here. Some of these organizations also provide counseling online and by telephone. Maintain good communication with your missionaries so that if a crisis strikes, they know that you are someone that they can trust to assist them in getting the help that they need.
Care packages are always welcome for those items from home that are most missed, but those items are different for everyone. Find out what items are particularly longed for by your missionaries. Sometimes these are a particular food or toiletry item. Sometimes it's a craft item, an office supply, or clothing item. Also while living overseas, a missionary can become disconnected from the current trends in Christian publishing and music. They frequently have a hard time keeping up with which new authors and artists are good. Buy your missionary a Kindle and an ipod and when you find an excellent new book, buy them the Kindle edition, doing the same with the ipod and good new music. Send recent releases in quality Christian dvds that they and their families would enjoy. Send over educational materials and good reading materials for their children as well.
Missionaries can also begin to feel disconnected from their home churches when they return after several years overseas to find that staff members or families in leadership have left or that there have been major changes within the church. Keep your missionaries aprised of what's going on within the church with regular newsletters a few times a year or when major changes occur. It's still their 'home church' even if they aren't there on Sunday mornings.
Missionaries always enjoy visits from their pastor or others on their church's leadership team, but they can enjoy those visits more when the visitors come with a heart to minister to the missionary and offer support and encouragement. See the Visiting Your Missionaries page for information about how to make the most of a trip to the missionaries your church supports.
Maintain a list of medical professionals within your fellowhsip who are willing to provide phone consultations with your missionaries. Encourage them to register with Missionary Telemedicine Organization to be available to other missionaries as well.
Maintain a list of educational specialists (those who specialize in speech and language therapy, occupational therapy, or special educational needs and those who can provide educational assessments) who can consult by phone or Skype with missionaries about any issues their children may be facing. Perhaps someone in your congregation would feel led to become an educational consultant by going through the training offered by the Professional Association of Cross-Cultural Consultants in Education. In addition to providing professionals who can help remotely, all missionaries with children should, when their children are pre-school aged and they are making decisions about their education and again when their children are middle-school aged and they are making decisions about high school, be offered the opportunity to attend a conference with either Share Education Services (in Europe), Anchor Education (in Africa), or Asia Education Resource Consortium (in Asia). I am unfortunately not aware of a similar organization in South America.
MINISTERING TO MKs:
The International Society of Missionary Kids has a wonderful article on their website giving ideas on how to minister to missionary kids. The organisation is a branch of the Assemblies of God churches, and therefore gives some ideas specific to their denomination, but the majority of the information is useful to all churches supporting missionaries with children. The missions blog A Life Overseas has two articles which would also be good to bear in mind when ministering to MKs: Questions Third Culture Kids (and Their Parents) Dread and Questions Third Culture Kids and Their Parents Love.
MindtheGaps.org has an excellent template for maintaining a list of resources within your church for missionaries.
A Life Overseas has a wonderful article with ideas for encouraging a missionary when they are overseas.