I have very low water pressure in my shower. The water pressure use to be good but has recently been getting lower and lower. It is almost a trickle now. The pressure in the rest of my house, including the bathroom sink, is good. Over the last few months the shower has been leaking from the faucet so I think I have to replace a washer or something. Any ideas about what the problem might be?

Do you live in an area with hard water? If you do then it may be a build up of calcium in the head and tube of the shower. I live in an area with very hard water and regularly get this problem. If this is the case then soak the head in a bowl with water and vinegar to help loosen the calcium and wash it through.

Check the flow with the shower head off.

The water in my area is not too hard and I have a water softener system in the house. The pressure is low at the shower faucet... even before it gets to the shower head.

The water in my area is not too hard and I have a water softener system in the house. The pressure is low at the shower faucet...even before it gets to the shower head.

Doug asked you to check the shower heade before I did, but that would have been my first response.
You have just isolated the problem to the shower valve and/or its supplies.
Turn the water off to the valve and open it up. Check and clean the inlet ports on the cartridge. Before you button up turn the water back on for aminute to flush out the supplies, (you should have a strong stream coming out of the open valve body) Reassemble and test. Better now? Regards, Tom