How Can the Pressure be Increased in a Shower?

If the shower is fed from a hot water tank, a cold water tank provides the pressure head to force out the hot water from the tank. This is usually in the loft to maximize the head so if it is located there, you are not going to be able to rise it any higher.
Cold water power showers are fed from the incoming mains cold water supply and the water is heated by a high powered electric element.
Shower heads incorporate a restrictor/aerator to reduce flow and water wastage. The Federal Energy Policy Act of 1992 made it a requirement that these were added to shower heads to reduce flow to about 2 gallons per minute, so removing this would increase the flow rate. Also it is possible to buy a shower head designed for a lower flow rate. This increases pressure but the flow rate will be less.
You can buy a booster pump which effectively works like a power washer and increases the pressure while keeping the flow rate constant.