If the 6 Volts is a solid requirement, then you need an LDO 5 volt regulator that draws as little quiescent current (i.e. current of it's own) as possible. If surface mount is not a problem, then try this (though it's only good to 250ma so it doesn't cover the full USB current range):
* MCP1703-5002E/DB Mouser Part #: 579-MCP1703-5002E/DB
A thru hole possibility is (but it's only good to 100ma):
* LP2950-50LPRE3 Mouser Part #: 595-LP2950-50LPRE3
These are ones that I have used in the past, if you need more current handling capability, then you'll need to search for something else (or perhaps someone else will chime in with a alternative
