Haiti literally means 'high mountain'. Except right on the coast, everything is on an incline- think San Francisco on steroids. At a minimum, you should walk hills or spend a lot of time on a stair-master prior to your visit. If you are going to be part of a construction trip, its a very good idea to add in some weight-lifting as well.
We'll put in a basic workout here. Whatever you do, make sure to have your doctor's permission first.