I just replaced my hot water tank and installed a new one. Now, the hot water pressure in the shower/bathtub (only) is weak - all other faucets seem normal, and the cold water in the shower is fine. Water pressure was fine before I replaced the tank, the problems with the old one were that the pilot kept going out, it was leaking from the bottom of the tank, and it was almost 20 yrs old. Any suggestions on how I can increase the hot water pressure to the shower/bathtub?

What type of shower valve do you have. First thing to check is for sediment that was disturbed in replacing the water heater and moved to the shower hot water valve.