There are a host of great free resources on the web for those helping send or for those going as missionaries long-term. Here's a list:

Missionary Care The most thorough list of articles and ebooks I've seen

Synergy Commons A global learning community for Christian leaders

Crosswired Solutions for the common problems missions communities have

Ask a Missionary Great web site with answers to your questions from experienced missionaries

Frontier Missions A free monthly online missions magazine

International therapists directory Christian therapists available around the world 

List of good books & articles 50+ of them in a pdf file

Ethne-Member Care A list of articles

Going Global: A Member Care Model for Best Practice
Stephanas: A New Testament Example of member care
Supporting Mission Workers The Key Role of the Sending Church

Sending Church Profile, Raising the Standard, Focus on Member Care Articles by Neal Pirolo

Historical Perspectives

For Everything There is a Season...And a Summons
To the Ends of the Earth
Wise as Doves and Innocent as Serpents In 12 languages

Giants Foxes Wolves, and Flies In 5 languages

Great Commission Resources Joshua Project

IJFM, Issue on Member Care
Doing Member Care Well How to send and care for missionaries

WEA Resources A list of articles and books available in Spanish and Arabic 

Missionary Care: Counting the Cost 25 chapters available to download

People In Aid Note their list of publications in Resources & Publications

Church Planting T4T Online training in how to plant churches