Word of advice though- even though it uses up a little more oxygen- if your lungs are in good enough shape singing always gets me up hills faster.
Ha! I sing to myself on hills (or running)-- usually "99 bottles of beer" for cadence. Sometimes outloud if I'm feeling the need.

As for sit vs. stand, I do both -- I guess kind of a cadence, how hard is it getting? sort of thing. If I'm slowing significantly and need the boost, I will stand. I'll often stand at the steeper areas, then sit to spin the rest .