Credit and my thanks go to Bill Lewis at
Increase Your Water Pressure
Most homes have a water meter that is smaller than the pipe feeding it from the street main. If that's the case at your house, you can replace your existing meter with a larger one. A common improvement is to go from a 1/2" meter to a 3/4" one. That's step one, and you will need to have a professional do it in most jurisdictions. Have the professional extend the 3/4" pipe through your foundation to a new 3/4" main shutoff valve while you're at it. That is the point at which you can start doing the work yourself.
The next trick is to carefully downsize the pipes at critical points as you continue through the house. Let's say you have a two-story house with the water heater in the basement, the kitchen and a small bathroom on the first floor, and a larger bathroom, or two bathrooms, upstairs.
Run 3/4" pipe to the water heater, which should be the first thing supplied in any home. Above the water heater's inlet fitting, install a 3/4" X 5/8" X 5/8" end tee. That is, a tee with those dimensions and the 3/4" opening on one arm of the tee, not on the base. Bring the hot water out of the heater in 5/8" pipe.
Continue both the hot and cold supply lines in 5/8" pipe to the next important split - say where the risers for the upstairs baths take off. Use 5/8" X 1/2" X 1/2" tees to make those splits. And so forth. Work down to pipe or tubing no larger than 3/8" before you connect to any faucet, toilet or other use.
If you pass non-critical points on your way to important splits, use a fitting that will keep the main flow going. For example, if you need to supply outside faucets while you're still in the basement, branch a 3/8" line off the 3/4" or 5/8" that you're running
At the end of each run, you should be feeding 1/2" into two 3/8" supply lines at the largest - or a 1/4"line for your icemaker of humidifier, possibly
One word of caution: open the taps slooowly when you first try the system after doing this!