What kind of difference will you make?
Another important thing to remember is to manage your expectations. Voluntourism trips are usually short -- two weeks or less. You can't expect to see big community changes within such a short timeframe. If you find it important to have closure at the end of your trip, make sure you choose a project that has a beginning and an end -- for example, building a well or painting a schoolhouse.
Experiencing a community markedly different from your own can be a heavy experience. Be prepared to process everything you deal with. You might see extreme poverty, unclean living conditions, malnutrition, drought or severe weather conditions. On a brighter note, you can also expect some time to yourself where you can be a regular tourist. Many voluntourism packages include time off for exploring. You might get a free day or two at either end of the trip, or you may only be required to work a few hours a day, with the rest of the time to yourself. Every trip is different, so choose one that suits your preferences.
Voluntourism seems like a lofty and charitable idea. However, the concept does have downsides.