Every culture has taboos. It's up to you to find out what they are. Something that seems innocuous to you may be insulting to someone else. American actor Cameron Diaz found this out the hard way in Peru when she appeared in public wearing a bag emblazoned with a Maoist political slogan. She didn't realize that the Maoist Shining Path had murdered 70,000 Peruvians in the 1980s and 1990s.
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.