Habitat for Humanity is one of my personal favorites. My husband and I spent a week on a build in Louisiana after Katrina and here are a few things I learned:
1) I learned that we cannot disconnect philanthropy from the economic injustices that cause the need for philanthropy. (Thanks MLK) We went on our Habitat Build in Louisianna a few years after Katrina and there were still hundreds of people living under highways, and hundreds of buildings still not re-built. I sense that if family business of the then president had been construction instead of oil, that Katrina areas would have been rebuilt in record time.
2) I learned a deep appreciation for the amount of human energy that builds a house. I've always had a love and appreciation of architecture and homes, and even participated in renovations. This gave me a front row seat to the amount of human labor that it takes to build a house from the ground up. Humbling and overwhelming. Hammer, hammer, hammer and then hammer some more. Holy, moley!
3) Take a palm nailer - there are a lot of great tools that can speed up the building process if you know about them. If you join a Habitat for Humanity build, I suggest you make a visit to your local hardware store, tell them what you're going to be doing. They can advise you on tools you can take to make you a power builder on your team.
4) Pack ear plugs and your favorite pain reliever. All that hammering is noisy and hard on your ear drums. And all that hammering and lifting will give you a few aches, so arrive prepared.