Taking a relaxing hot shower is just what the doctor ordered at the end of a long day. Few things can be more irritating at this moment than a shower with insufficient pressure. One common cause of low pressure is a leaky diverter valve located in the spout. When you pull up on the knob on the spout, it blocks water from leaving the spout and diverts it to the shower head. A malfunctioning diverter valve will let excessive amount of water through, creating low pressure and a leaky spout. A full replacement of the spout is usually the only fix.
Point a flashlight into the shower spout and look for anything that might be blocking the diverter valve. Dirt, hair and other debris can cause the valve to jam and cause a leak. Removing the block will return the valve to full working condition.
Turn off the water to the house if there's no blockage -- the diverter valve may be broken or eroded due to normal wear and tear. This means you'll have to remove the spout to fix the problem. You should shut off the water any time you remove a plumbing fixture.
Remove the spout with either a screwdriver, Allen wrench (otherwise known as a hex wrench) or a plumber's wrench if the spout is threaded into the wall. Keep all the screws in a safe place.
Take your spout to a hardware store and compare it to other spouts of the same size and shape. Spouts come in all shapes and sizes, some of them very similar. By bringing the spout to the hardware store, you avoid a return trip because you grabbed the wrong size.
Attach the new spout with the tool you used to remove the old spout. Turn the water back on and test the new diverter valve. It should work like new and greatly increase the water pressure coming from the shower head.