While letters which are packed full of information is greatly appreciated, small birthday and holiday cards with a short personal greeting are beloved as well. Many missionaries have cassette players, and might even have video players. Hearing the voice of a friend can be very touching. Don't just record a greeting, be creative. Sing a song, or read a poem or a passage of Scripture which has been important to you recently.
Phone calls are also a popular encouragement.
One word of caution. Some missionaries are in closed countries, e.g. countries that don't welcome missionaries. Find out what communication guidelines are suggested before you send something. The Missions Team can help you learn the appropriate guidelines. Material Support Provide for their finance support.