Budget Travel has come up with a list of countries Americans need visas to travel too and the best ways to obtain them.
For example, Kenya: Get a visa for $50 (U.S. cash only) at the airport upon arrival. Forms are available at the airport.
For some other destinations (Brazil, China, Russia) it may be best to hire the services of a professional to avoid any complications. I’ve been to several places off the beaten path and in most cases you can do it yourself.