How To Improve the Water Pressure in your Shower Head?

Low pressure can be caused by old galvanized pipe, which often build up deposits causing the water to be restricted.

Another cause of low pressure can be incorrect pipe size. I like to run a 3/4" trunk line with 1/2" legs to fixtures (which I think is pretty common).

Some houses have pressure reducing valves on the supply line, which are used to control the water pressure inside the house.

For those pointing out that this trick actually increases flow rate, and not water pressure, you are correct, but I think this is a common disconnect between the lay-person's vocabulary and the professional's terminology.

If you have an issue where someone flushes the toilet, or turns on the shower/sprinkler/etc and you can hardly get any water out of a second faucet, many people would say "this water pressure sucks" but what they actually mean is, this house has terrible flow rate.
My 1940's cottage, for example has water pressure of about 103 psi (which is pretty high), but has old corroded pipes under the house that have severely restricted the flow rate of this properly pressurized water. As a result, I can get enough water from any of my high pressure shower heads, but trying to run multiples is very difficult.

If you're on city water (wells are a different story) it is much, much more common to have flow problems (not the city's responsibility) than pressure problems (at least partially the cities responsibility). So buy a pressure gauge from your local store for $5 and check your pressure, if it's the problem, call the city utilities dept and ask what they can do to help fix it, that's what your water bill and tax dollars help pay for anyway.