The water pressure is a function of a couple different things, but if you have a pressure tank, and you have a lot of old iron pipe, you may have a situation where the pipes have become corroded. Look for places in your mainline where the pipe is unusually small. For example, the house has 1" pipe, but right near the water tank, there is a piece of 3/4" pipe. Check those locations first for corrosion/deposit that has built up in the pipe.
Finally, make sure your water tank isn't completely jambed with hardwater deposits, as well.