A quality-built consumer air compressor can keep working hour after hour with little attention. However, at the end of the day, when you have finished using your compressor, there are a few steps you should take to ensure its long operating life. Here is what you should do:

1. Turn off the master switch and unplug the compressor - Most compressors will automatically recharge their tank when the air pressure drops below a preset point. If you leave your compressor on, it is likely to start and stop by itself; this is not only wasteful, it is alarming when your compressor suddenly powers on at 3 AM when everyone is sound asleep inside. In addition, you should also unplug the compressor to prevent children from flipping on the power; this also protects the machine's electronic circuitry from power surges.
2. Properly store your air hose - Your air hose is a vital link between the compressor and your pneumatic tools. If you mistreat it by leaving it on the shop or garage floor, it will be smashed, cut, pricked, and eventually become a leaky, unreliable piece of equipment. Take care of your air hose by disconnecting it from tools and coiling it up neatly. Store it on or near the compressor, well above the floor of the shop or garage.
3. Disconnect, clean and lubricate your tools - Pneumatic tools are rugged, but they need attention, too. You should always disconnect your tools from the air hose at the end of the day to allow any internal moisture to evaporate. Also, be sure to wipe your tools down with a clean, dry cloth to remove dirt, grease, or any foreign substances. If your tools seem to need lubricating, apply a lubricant designed for use with air compressors. Never use any type of non-compressor oil or grease on pneumatic tools.
4. Empty air from the tank and drain accumulated water - All compressors will accumulate moisture as incoming air releases water in the form of condensation inside the tank. This water is very detrimental to your tank's integrity and can cause rusting. A rusty tank is prone to leaks, and it can even fail catastrophically. 

To counter accumulated water, most air compressors have a drain valve near the bottom of the air tank. You should open this valve at the end of the day to release accumulated moisture. To drain the tank, simply use a pair of pliers to slowly open the drain valve. Be sure to keep your face away from the valve as the rush of air can be strong; you don't want a foreign object to be blown into your eyes. Open the valve all the way, but don't twist too tightly to avoid damaging the valve. Allow all the tank's air to escape, and you should see water spraying and dripping from the tank, too. Once the air is gone and the tank stops dripping water, close the valve with the pliers, again taking care not to over-twist. Need more information about air compressors? Find out here.