From the US Dept of Energy:

Programmable Thermostats Save $: You can save around 10% a year on your heating and cooling bills by simply turning your thermostat back 10°–15° for eight hours. You can do this automatically without sacrificing comfort by installing an automatic setback or programmable thermostat.

When programming your thermostat, consider when you normally go to sleep and wake up. If you prefer to sleep at a cooler temperature during the winter, you might want to start the temperature setback a bit ahead of the time you actually go to bed; you probably won't notice the house cooling off as you prepare for bed. Also consider the schedules of everyone in the household; is there a time during the day when the house is unoccupied for four hours or more? If so, it makes sense to adjust the temperature during those periods. From Energy Star: Programmable thermostats maximize the money saving abilities of new HVAC equipment: It's a good idea to upgrade an old manual thermostat to a programmable unit if you're replacing a CAC or heating system given that programmable thermostats are far more accurate and will maximize the efficiency of your new system.