Technically, they do not. The 27th Amendment (submitted for ratification 1789; adopted in 1992) states that any changes to Congressional compensation doesn't take effect until after the next general election.

http://en.wikipedia.org/wiki/Twenty-seventh_Amendment_to_the...

My understanding sit hat Congress has the authority over the salaries of the congress following the next election. If they abuse it too horribly, that's not going to include them.