Connections that rely on threaded pipes and fittings are prone to leaks if they're not sealed with either Teflon tape or Teflon pipe joint compound. Careful plumbers use both on every joint for extra security. They don't want to come back.
Start by wrapping the male threads with Teflon tape (Photo 1). With the end of the threaded pipe facing you as shown, wrap the tape clockwise. Usually three layers is enough. Once in a while, you'll run into a loose fitting that requires four or five wraps. Stretch and tear the tape to complete the wrap.
Spread a thin layer of Teflon pipe joint compound over the tape (Photo 2). If you're working with plastic pipe, choose Teflon pipe joint compound that's compatible with it. Then start the threads by hand before tightening the connection with wrenches (Photo 3). Wipe away the excess.
Compression joints are most common on shutoff valves, although you find them on other fittings as well. They have a brass or plastic ring (ferrule) that's compressed into a recess when you tighten the nut, forming a seal. Lubricating the pipe and the ferrule with a bit of Teflon pipe joint compound (Photo 1) helps the ferrule slide along the pipe and squeeze tightly into the recessed fitting with less wrench pressure (Photo 2). Tighten compression fittings firmly with two wrenches to crimp the ferrule onto the pipe (Photo 3). Also make sure the pipe or tube goes straight into the fitting. Misalignment will cause a leak. If the fitting leaks after you turn on the water, try tightening the nut an additional one-quarter turn. This usually stops the leak.
Joints on chrome trap assemblies rely on rubber slip joint washers for the seal, which often leak. If you're reassembling a chrome trap, buy new slip joint washers and nuts. However, new washers sometimes stick to the pipe, causing them to twist or distort as you push them tight with the slip joint nut. To avoid this, lubricate the drain tubing and slip joint with a little pipe joint compound (Photo 1). The compound helps the washer slide smoothly and creates a tighter seal.
Start the slip joint nut by hand, and twist it on until the threads are engaged correctly. Hand-tighten all joints first (Photo 2). Then adjust the trap parts until they're aligned and pitched slightly for drainage. This is key; a misaligned joint will leak, even with new washers. Finally, use a large slip joint pliers to tighten the nuts an additional half turn.
Plastic trap parts use hard plastic slip joint washers for a seal. Make sure the flat part is against the nut with the tapered side facing the fitting.
The skinny copper or chrome supply tubes used to connect faucets and toilets (Photo 1) are tricky to cut, bend and align. But you don't have to put up with them. When you're replacing a faucet or toilet, use flexible supply hoses with a braided covering instead (Photo 2). They have rubber gaskets at each end and don't require much force to seal. They're available in many lengths and are flexible enough to fit almost any configuration. The only trick is buying a connector with the correct size nuts on the ends. Take your old tubing and the nuts on each end along with you to the store to be sure of an exact match.
Start the nuts carefully and hand-tighten. Then tighten an additional half turn (Photo 2). Avoid overtightening. It's easy to tighten the nuts a little more if the joint leaks.