There are also classic fundraisers like candy bars and cookie dough fundraisers, where you send your missionaries out into the community to raise money. But if you need more significant help and prefer to approach people on a personal level, writing a mission trip fundraising letter can be a more appropriate way to go.
When writing a mission trip fundraising letter, be aware that this is going to take a significant amount of time and effort. After all, you only get one shot, so you want to make sure you send out the best possible message you can, so that you appeal to the greatest amount of people possible, and get a positive response in as many cases as you can.
Remember that you are asking people to give their money to the mission trip, so you need to phrase that in a way that makes them see the benefits. The first thing you want to do is thank your supporters for their past help, because most people who contribute to any cause do like to have that simple recognition. Don't take too much time getting to the point - you don't want your mission trip fundraiser letter to go on forever and bore people to death. State what you're doing, why you're doing it, and how much is needed in total.
Let your supporters know that any help they can give would be appreciated, but don't come across as begging. Most of all, make your mission trip fundraising letter easy for your supporters to respond to - include an addressed, stamped envelope where they can simply enclose a check and send it in.