Install a programmable Thermostat

Most are easy to install, if you're handy.  They can be purchased at home improvement centers, and prices start at $50 for a simple unit with a timer.  Set the thermostat to turn the heat back on at night & during the day when you're at work.  You can set it to start warming the house back up a half hour before you get up or return home, so the house will be cozy when you need it to be.