In the OED entry for the verb "speak," there is a sub entry for "to speak for." About half of that is devoted to "speak for yourself," which is what I quoted. It is followed by the typical list of examples.

There was a Simpsons reincarnation joke that used the phrase:

Sideshow Mel: You only live once! Apu: Hey, speak for yourself!

http://en.wikiquote.org/wiki/The_Simpsons/Season_9#Miracle_o...