Main water cutoff:
Make sure everyone in the house knows where it is in case of an emergency.
 

Toilet overflow:
If you believe your toilet is about to overflow, quickly remove the tank lid, reach down and push the flush valve closed. It covers the tennis ball-sized opening in the bottom of the tank. That keeps the bowl from overflowing while you clear the obstruction.
 

Toilet Check:
Remove tank lid, add food coloring to the water in the tank. Check the bowl in ten or fifteen minutes. If you see color, the flapper valve may be leaking. This should be done every few years. Most such leaks are silent.
 

Check your meter periodically.
If no water is running in the house, all dials on the meter should be stable. A movement in one of the dials may indicate a water leak, probably in a toilet. This could cost you dearly over time.
 

Stop valves:
These are the cutoff valves near each toilet and faucet. Familiarize yourself with their location, so that if a major problem develops you can turn the water off there instead of at the main to prevent serious water damage to your home.
 

Open and close all valves once a year:
This will ensure that they will work when you need them to. Otherwise they may seize up over time.
 

Water heaters:
Their life can be extended by periodically draining a gallon or two of water once or twice a year from the drain valve at the bottom of the heater. This will allow sediment to clear.
 

Stopped Drains:
Use drain-cleaners as a last resort, because they often don't work, and if they don't you will have to deal with a sink full of caustic liquid. Always use a plunger first, and if you have to call a plumber after using drain cleaner, be sure to let him know.
 

Plunging:
The best plunger to use in a sink drain is one that has a neck on it.
 

Laundry valves:
These should be closed when not in use in case hose breaks.
 

When buying fixtures:
Be sure replacement parts will be readily available.