Add/Use a Programmable Thermostat.
Do you really need to heat or cool your home when you are not there? However, you would really like it to be comfortable when you come home from work. Add a programmable thermostat and learn how to use it. Vacation holds are as simple as holding a temperature for a certain number of days then telling the system to kick on a few hours before your return.
Note: Do not let your home temperature drop below 55F in the winter or you will be repairing some busted pipes and cleaning up a flood.