Inspect and repair caulking and/or weather-stripping around windows.
Prep your heating and/or cooling systems for each season: Clean your window-mounted air conditioner's air filter. Clear debris from your central air conditioning system's condenser coils. Replace your furnace or air handler's air filter every six months or more frequently, depending on your home's particular needs.
Use a ceiling fan in lieu of your AC, especially at night and when temperatures drop. Similarly, using lightweight bedding and wearing clothes designed for warmer temperatures may also reduce need for air conditioning systems during evening hours.
Clean your dryer's lint filter after every load and annually, clean out the entire venting system with a vent cleaning brush. Pay attention to increased drying times, which may indicate lint buildup in the venting. For gas dryers, consider installing an alert system to detect an air flow restriction.
Close window drapes and shades to deter fast-warming rays of sunlight from reaching your rooms. This may reduce the room temperature by at least a few degrees, taking a load off of your AC and saving energy.
Monitor appliance and electronic equipment energy consumption with an energy meter. Discover inefficiencies and determine what needs to be unplugged or replaced.
Use slightly cooler water. Reduce your water heater's regular temperature setting. If you're away for an extended period, be sure to use its vacation setting. Use warm or cold water in your washing machine rather than hot.