Tip: Gate Valve and Stopcock Maintenance
Exercise your gate valves at least once a year. You don't want to have to un-seize them in an emergency! Simply screw the valve completely off, then open it fully again. If a valve won't shut off, try giving the body lots of sharp taps (don't hit it hard). This may free the shaft(spindle) and gate (the shaped brass bit which actually shuts off water) from the body. If that doesn't work, try heating the body with a hair drier. Turn the handle of the valve before the body cools. Lastly try using a stilsons or pipe wrench on the handle of the valve to get extra leverage. Use e.g. a vise grips on the body of the valve to exert a counter force. This reduces stress on connected pipework. Don't use excessive force on the handle as you can quite possibly wring the shaft of the valve.
Checkout my guide for valve locations in the home here:
(re: Cleaning tips challenge- http://www.hometalk.com/diy/e-cleaning-tips-challenge-26729461)