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Since the original link is long dead now, I am copying this over from BFs forums.

First of all, let's talk about the instruments.
Besides the Pitot and static instruments, we shall focus on the engine section.
In order of importance we have:

1. Manifold Pressure
2. Tachometer
3. Carb. Air Temp
4. Coolant temp.
5. The multi-function engine gauge

The various models have these gauges in different locations, but a zoom in will reveal their location on the panel.
For ME, the most important gauges are Manifold pressure and Propeller RPM/Governor RPM. Without a proper setting on either, you are wasting E and fuel.

Right up front I must explain the psychological aspect of airspeed.
Most of us want top speed RIGHT NOW. If you fall into this category, you might consider moving along to a different aircraft.
The P-51 accelerates well from zero to about 200MPH in most flight regimes. Try to take him over that in a hurry will reward you with a hot engine and perhaps death. Since the acceleration is so gradual in the Pony, most consider him to be a DOG in the sky. I suppose he is compared to the HotRods like the Spit, 109, LA5 etc. The huge difference is that the P51 is a high wing-loaded fighter that is slippery as hell at high speeds. When you get him up to high speed, he holds E very well unless you abuse him.
Having said all this, let's get to the meat of the matter.

I suggest you take a test hop in a P-51D-20NA to start off with. I suggest 100% fuel with no droptanks. Take off at 100% power (WEP) is useless on the deck).
As soon as you clean up (Raise gear and flaps) adjust rudder and elevator trim for straight and level flight. Adjust the throttle to 50" Manifold Pressure (There is a handy red line on the gauge) and set the RPM to 2700. (Top end of the green band)
Adjust your elevator trim for 1500 feet per minute rate of climb and keep the 'Ball' on the turn/bank indicator in the center. The moment the airspeed picks up, you will notice a pitch-up of the nose as well as a skid/slip opposite of torque. Keep fiddling with the two trims to maintain the 1500' FPM climb.
As you pass through about 12,500' ASL, you will see the manifold pressure reading drop drastically. This is the automatic supercharger changing to High Blower. Since an abrupt change would cause detonation and destruction of the pistons, the supercharger wastegate is opened to keep the boost levels reasonable. If you add power, the gauge will climb, then pop back to a lower setting. This is normal. The trick is to be patient and let the aircraft climb up into it's most effective flight envelope. (Maxing out around 25,000ASL) Once you get up to around 15k, you will notice the manifold pressure climbing until you must reduce throttle or go into overboost. (More than 50" pressure)

Now, having flown up that high a few times, you can start to get a feel for the different 'Sweet Spots' the Mustang has.
To make the best use of the P51s abilities, you must understand that patience is critical. If you want to get up to 450MPH, you are going to have to earn it. Think of the Pony as a medium weight automobile with tons of horsepower, but little torque. You can accelerate well for a bit, but then the load is too much for the engine. To advance further, you must change gears (Prop RPM) and maintain a reasonable power setting. The trick is to level out, set power for 50" pressure, 2700 RPM prop and keep him straight. Over the period of about 3-5 minutes suddenly you are going 350MPH. You can roll the power back to 40" pressure at 2700RPM and watch the airplane accelerate even more. If you choose to pull some pitch, beware that the airspeed will drop quickly. the solution there is to bring prop pitch up gradually as you pull up, while adding power up to the redline. If you do this too soon, you waste a few 'Gears' in between.
If you find yourself running WEP with prop RPM over 2800 and not taking off or in a weeds turn fight, you are wasting power and fuel.
You will discover that 2700RPM is the sweet spot for the prop. To get this, you may need to have a 75% throttle at 18k, but 90 on the deck.
The way the 'Stang was designed was radically different from previous aircraft. The pilot selects the power setting and prop RPM, and the controls took care of the rest. If the pilot wants endurance, he chooses say 65% power and 2500RPM, which will take him from Bristol to Berlin and back with fuel left over. If he wants max performance, he chooses a power setting for 2700/50+" pressure and watches the carb air temp as well as coolant temp. Keeping the Pony too long with either level too high will actually slow him down to the point at which he is a BRICK in the sky.

^^
I used 90-95 PP on the deck.