In climates where temperatures drop way below freezing, most newer homes have “freeze-proof” faucets. The trouble with these faucets is that water trickles out for a few seconds after the valve is closed. That trickle makes you think the valve is still open, so you turn the faucet handle harder. It doesn't take much of this repeated overtightening to wreck the valve's rubber washer.
Luckily, most of these faucets are almost as easy to fix as they are to damage, and the replacement parts are inexpensive. Your faucet may look a little different from the one we show, but the basic steps will be the same, even if you have an anti-siphon faucet (see photo above). In fact, you can fix most outdoor faucets that aren't freeze-proof the same way.
Photos 1 - 3 show how it's done. Don't forget to shut off the water supply before you get started. Be sure to hold back the faucet when you unscrew the packing nut and later screw it back on (Photo 1). Otherwise, you may unscrew the threaded joint inside the house. If your stem has rubber O-rings on it, replace them as well as the washer. The best way to get the correct O-rings and washer is to take them with you to the hardware store or home center to get an exact match.