The last few years, my dad's little project was to reduce our family's energy footprint. We've reduced our usage so far that the majority of our PG&E bill is the connection fee we have to pay every month (The average bill runs about $25). This wasn't one big thing (except maybe the solar panels), but many little things, which I think I might share with my fellow apartment dwelling friends so they too can strive to reduce their PG&E bill.
First one will be an easy one. Set your AC/heater less agressively. Running the heater at 70 instead of 75 makes a big difference.
Can't really say this one helps me much. We have turned the AC on once in the past two months, and that was because there was a dozen people in the living room. I have better ones on the list, I promise.