Why and How to Change Filters

Dirty filters kill your AC's efficiency, so install a new one every month during the cooling season for central and window units (or clean them if you've got the washable type). Look for the filter's minimum efficiency reporting value, or MERV, which ranges from 1 to 12 for home AC units; the higher the number, the better filtration it provides (and the more energy needed to pull air through it, so balance air-quality concerns with energy costs).

Why and How to Use a Timer

There's no need to blast the AC while you're away. For central units, install a programmable thermostat that lets you set higher temps when you're gone and cooler temps when you're home. Newer window units have built-in timers and adjustable thermostats, or you can buy a timer at any home store for $10 to $20; just make sure it matches your device's voltage. Unless you're going on vacation, don't shut off the system; otherwise, the air compressor will need to work harder to cool your house later.

Why and How to Fix Duct Leaks

Ducts can lose up to 30 percent of airflow through leaks. Find leaks using the old "smoke trick": For window units, light a stick of incense and hold it where the unit and the window frame meet; for central AC, hold the stick near duct connections. If the smoke blows around, you've got leakage. For ductwork, use foil tape to seal small gaps and duct mastic for larger ones; for window AC units, stuff foam between the device and the window frame, taping as needed.