If you don't have a programmable thermostat, then buy and install one!

Hopefully, you do have this energy saving device already installed, but that is only half of the battle.  The programmable thermostat does no good just sitting on the wall looking pretty.  It actually requires you to use your index finger (or one of your other digits/opposable thumb) to dial in the settings!

This sounds obvious, but think about it:  Do you currently have your programmable thermostat dialed in to the setting that enables you to stay comfortable while using the least amount of energy?

Winter:

Do you set it to be be cooler at night in areas you won't be sleeping in.  Do you make it cooler during the day when you are at work, and have it set so the heat kicks on right before you get home?

Summer:

Do you set it just cool enough at night to be sleep comfortably with your ceiling fan on. (Side note: Before I installed our ceiling fan, I had to set my AC on 72 deg. F to sleep comfortably.  Once I installed my ceiling fan, I could turn that on medium and set the thermostat on 75 deg. F.  Yes, the energy saved on AC is greater than the energy the ceiling fan uses)

The goal of the programmable thermostat is to cause your AC or Heat to come on as little as possible (duh, I know).  Think of it as a game, a game in the real world you can make money and help the environment at.