How can I correct low hot water pressure in bathrooms?

The most common cause of low hot water pressure is debris building up in the heater and partially clogging the mixing valves. If I'm right and this is your situation you will need to do several things starting with flushing the heater. Do not shut down the main, just add a hose to the drain at the bottom of the tank and let it flush for several minutes into an appropriate drain in the floor. Now you need to shut the main down and open each mixing valve and clean them. While the valve is open put a towel over the valve body and hold tight while someone else opens and closes the main, this will flush the lines. Let me know how this works for you. Bob