Consider a 'RTW' or 'Round the World' airline ticket. Many
international carriers have formed alliances / partnerships with each
other (code share) that helps keep the cost down. Many offer almost
unlimited stops on your RTW journey *so long as you do not back-
track*, i.e. you keep travelling in an easterly (for example)
direction.

See: http://www.airtreks.com/

or

http://www.lostluggagetales.com/reso...-tickets.shtml

for additional insight.

MW