1) Put together a prayer meeting
It could be something that meets on a regular basis or maybe a special one time thing, but either way, it's special. If the missionary is not able to attend, send an email asking for special prayer requests. It might not sound too different from the prayer you do everyday, but trust me, it would be a huge encouragement to know that there are people gathered together to pray.
2) Host a Visiting Missionary
Missionaries travel quite a bit, and the costs of hotels and eating out really adds up. Offering to host a missionary is a great way to be a blessing! Whether it's for a meal or for the night, this is a great way to not only provide food/a place to sleep, but also to get to know them. Let your church (and maybe a few neighboring ones) know that you are willing to host missionaries when they come.
Speaking of travelling expenses...sometimes missionaries need transport. If a missionary is flying in they may need a ride to a from the airport. Do you have an extra vehicle or one that you could get by without? A friend who is a missionary in Senegal had someone lend her a car for a year and a half while she was in the States speaking to churches and going to seminary. That's a pretty cool!
4) Offer Your Professional Services
Here is where an hour or so of your time can be like a $100+ donation! Is there something you do that a missionary might need? What you do for a living (or as a hobby) might just translate into a huge answer to prayer. After coming back from Honduras I decided that if I took photos of missionaries (WGM or not) for their prayer cards, I would not charge them anything. Anything from free health checkups to fixing their computer to helping them make a really cool video...the possibilities are pretty much endless.
5) Be Their Champion
This is the big one. The huge one really. A champion comes alongside the missionary, and works with them to see that the ministry is funded and prayed for. They pray for them and support them financially, but they take it deeper. They tell their church and their friends. They share what they know about the ministry with who they know, and they get others on board. They get creative and do things like have a dinner for some friends and invite the missionary over to share with their friends. Being a champion doesn't mean being a superhero. It means being someone who is moved to action.