Here are some key thoughts that will help you on your way:

1. Choose who your support wisely. Do it in conjunction with your local church's commitments. You'll want to communicate your support commitment with the missionary or ministry or supporting. And you want to choose what amount and the timing so that you will be able to sustain it for the long-haul.
2. Keep in good communication with the person or ministry you're supporting. You don't have to wait for them to initiate an exchange of communication before you let them know you're praying for them and appreciate their ministry. When they do send out a newsletter, it's always a good thing to respond with at least a brief word of acknowledgment or encouragement. Like, "thanks for bringing me up-to-date."
3. If you're able, it's always a good thing to remember special calendar events, like birthdays, anniversaries, Christmas, special events for the ministry, etc.
4. Take the opportunity when your missionary or ministry representative is visiting in the area to be hospitable. Meet them for coffee or by them lunch.
5. Definitely let them know if there is any change in your support level, frequency, or ongoing commitment. They would want to know if you've lost your job or have a significant change in your income. It's always good for you to initiate letting them know why there might be some change rather than having the embarrassing obligation to ask you what's going on.