Buy a programmable thermostat
Growing up, you probably had a frugal relative who enforced strict limits on how high or low the thermostat could be set. If you don't have one now, a programmable thermostat can play this role for your home automatically. Based on your family's schedule, you can program it to automatically set the target temperature higher in the summer and lower in the winter when your family won't be home.
Prices on the thermostats have come down so much -- you can buy one at a hardware store for as little as $25 -- that installing one is a no-brainer, especially because the EPA estimates the average homeowner can save $180 per year with a properly programmed unit.