For the most part, showers are like faucets.  One’s horizontal and one’s vertical, and the parts in a shower are bigger.  Otherwise, the same principles and procedures apply.

Low Water Pressure

If it’s gotten lower over time, it’s the shower head.  Unscrew it from the shower pipe, then take it apart and find what’s clogged.  It might be like an aerator in that it’ll have a water flow constrictor whose tiny holes are filled with sand.  Remove it if you can.

If it’s just low in general, it probably has a water constrictor in it.  Remove the shower head from the pipe and look for a (usually white) piece of plastic with a small hole in it restricting the incoming water.  Remove it however you can.  If it’s permanent, drill the bastard out.

Dripping

Here’s the standard routine:

Determine which side is dripping.  It’s usually the hot side.  If the cold side seems to be working perfectly, leave it alone.

Turn off the water with the valve outside the house.

Pry the little plastic cover off the top of the knob.  Unscrew the bolt.

If the shower’s really ancient and crusty and has metal handles, you’ll probably need to pry on it from below with a couple of screwdrivers.  There’s also a special faucet handle-pulling wrench at the store.  Rock it back and forth in all directions and then give it a try.

Unscrew the valve with the crescent wrench.  Some valve stems are buried deeper in the wall and need a deep-well socket.  A regular deep-well socket probably won’t work, but you can try.  If not, head to the plumbing department and buy one of the deep-well sets made for shower valves.

Remove the screw from the end and replace the washer.

Stick your finger in the valve socket and feel the rim of the brass ring the faucet washer seats against.  If you feel a tiny nick, it’ll have to be ground down or replaced or it’ll eat up the new rubber washer in no time.

There’s a fairly cheap tool at the hardware store for removing valve seats, and there are only a few sizes of them, so pick up a new seat and you’re good to go.  It the circular seat has a square inside, it’s removable.

In older showers, the valve seats aren’t removable.  The store sells a ‘valve seat grinding tool’ that you can either operate by hand or use with a drill gun and this’ll grind the valve seat down until it’s smooth again.

Replacing

Actually replacing the whole valve assembly is usually above the average person’s skills, as it often involves soldering brass pipes with a propane torch, and that’s not to mention replacing the tile or fiberglass shower enclosure afterward.  If possible, the proper procedure is to go in through the sheet rock of the adjoining wall and replace it from behind.

Tubs

Dripping tubs are treated just like shower valves.

If the tub leaks, remove the overflow plate and see if it’s adjustable.  If so, lengthen it.  Otherwise, see if it’ll pull out.  Bring it to the store for a precise match and replace it.  It can be a bitch getting the little ‘bucket’ back in the hole, but just keep at it and eventually it should slip in.