Find the Most Inexpensive Airfare
Few organizations provide airfare, which is another major expense. However, for the younger volunteer (18-26) discount fares are available through STA Travel in both North America and Europe.
Despegar is the Latin American equivalent to Hotwire or Travelocity, and has great deals if you plan well in advance, including special fares for students and seniors.
FrugalMonkey is a partner site, and includes information on last minute travel options plus travel guides for several countries, including South Africa, Argentina, and Mexico. One tip when traveling to South America is to check out flights from Miami, which is a major transfer point for many major South American destinations. It can often be less expensive to book flights from your home city to Miami and another from Miami onward.
Taca Airlines features some of the lowest Latin American fares on the Web (often because of multi-leg flights) to major cities in the United States, including Miami. Of course, there are lots of other airlines to look into as well.
You can use a similar method booking trips from Los Angeles to many Asian destinations, using sites like Bezerk.com which searches multiple travel sites at once. Travel to Africa is still very expensive, but sites like Kayak.com offer some of the best rates.