In Soviet Russia, cars pan you. So if you're from Soviet Russia, you can stop reading now. If you're not, then you're here to learn how to pan like a champion. Panning is a technique which can easily be mastered with practice, but practice means nothing when you're doing it wrong.
Tip #1145: Stay the hell away from Soviet Russia.
Mentally preparing yourself
Let's begin. Close your eyes and imagine freezing the car with your hands. Now open your eyes and watch the track closely. Notice where the cars are coming from and where they are headed. When the car comes by on the right or left side, begin to follow it with your eyes. Don't touch your camera yet. You're not ready. 
Now that you've practiced following the car with your eyes, hold your hands out with your palms facing the sky, shoulder width apart. This is very important. When the car comes by again, try to cup the car in your palms like you're catching rain falling from the sky.
It might help to keep your shoulders locked and your hips loose, but make sure you plant your feet into the ground like a 150 year old redwood. This is an important and overlooked aspect of panning. Have your buddy come up from in front of you and push you. If you can overcome the backward force with your strongly planted lower body, you, my friend, are ready to begin panning. 
Pick your camera up and hold it in your right hand. Raise your left hand into the air like somebody asked a question and you're an eager student wanting to answer it. Stretch it out and then lower it and grasp the front end of your telephoto. Make sure you have a good grip on the lens with the left hand and a tight grip on the right hand.
* Focusing and camera settings
The first and arguably the most important setting is to turn on the AI SERVO focusing mode. AI SERVO or SERVO focusing is a mode where the lens motor will keep working while the shutter is half pressed (or the * button is pressed as we will learn soon). This is incredibly important because the cars are moving and regular focusing will lead to many misfocused shots.
Also, this might be a good time to mention that you should only be using one focusing point. Turning on all 100 focusing points sounds like a good idea if you don't know what you're doing, but this is in fact a bad idea! The key here is to reduce the amount of guesswork that your camera/lens has to do. If you are tracking a car and you're using multiple focusing points your lens may jump from the car to the background to the car etc. By using one focusing point you are eliminating a variable which might ruin some of your good pans. Keep it simple, use a single point.
If you haven't done so already, it might be important to switch your focusing to a button on the back of the camera. This is important for two reasons. The first reason is because you can pre-focus. Pre-focusing is a technique where you will focus somewhere on the track before (hence the prefix "pre" which means before). Keep in mind when you are focusing with the * button, the camera only focuses when you press the button so you can pan without focusing.
Motorsports are one of the most predictable sports in the entire world. You know exactly where the car will be this lap, the next lap, and the lap after that. You can predict with sound judgment how fast the car will be traveling seconds and sometimes even minutes before it arrives at your designated spot. For this reason, using a back focusing button has benefits that are two fold. One: You don't even have to focus when the car comes by and secondly, if you do have to pre-focus you are cutting the distance that the focusing motor needs to travel.
The second benefit of using the back button to focus (usually the * button) is that you will maintain a better grasp of the camera grip. By focusing with your thumb on the rear of the camera, you are exerting a forward force on the camera so when you do press the shutter button you are not pressing the shutter as much as squeezing it down. This is very important because by squeezing the button you are reducing the amount of downward motion a.k.a. JERK that the camera experiences when you finally take your picture. By reducing the amount of jerk, you will minimize the amount of photos deemed recycle bin worthy due to vertical axis blurring.
Let's do the damn thing!
Ok, now that we understand the advantages of pre-focusing and focusing modes/points, it's time to begin. Remember the redwood? We're going to plant our feet again with your hips loose and our shoulders tight. Swing your upper body from right to left a few times to get a feel for it. You are a pano head now and your tripod legs are, well, your legs.
So start pressing the button and take some pictures!
Not so fast 
Tracking
Before we can take a picture we have to understand tracking. Cupping the car in your hands was a silly exercise that made no sense at all. It's still not going to make sense now, but I'm sure it was hilarious for people around you when you were doing it. 
But not everything was a complete joke.
3, 2, 1.. go!
The red box is the goal here. For now, we'll focus on panning shots where the car is traveling perfectly perpendicular to you (this is where it is the easiest to nail a pan, you can deviate from this later). If the car is traveling from left to right, you will begin to track the car from 3 and then take your picture at 2.
Since we're taking the picture at 2, you probably want to pre-focus here. Hold your camera up to your face and wait for the car to approach point 3. You should already be looking into your viewfinder with your camera prefocused to point 2. DON'T PRESS THE * BUTTON YET!!! The trick is to NOT jump the gun. That would negate all our hard work! When the car approaches the *, that's when you press *. Don't waste your batteries and energy, just follow the car from point 3 to point 2.
Canon 20D | 200mm, 1/80s, f/16, ISO 100
TIP! Find a point on the car that stands out (i.e. headlight, grill, emblem, etc). Try to keep one of your focusing boxes exactly on this element and keep the speed of your pan perfectly aligned with the car!
Ok, so we're focusing and... car reaches point two..
PRESS THE SHUTTER!
.
The reason why we track the car is because it helps you calibrate your hips to the speed of the car. In this way, the camera is moving perfectly with the car so in theory the car will not be blurred, but the background will.
The X Factor
Now you've got the panning technique down. What about shutter speed, aperture, ISO, everything? Forget about all of that a second.
Keep in mind, you're attempting to blur the background not the car. For this reason, a faster shutter speed will mean more keepers because it will be easier to freeze the car. The downside? It will be easier to freeze the background too.
So if a fast shutter speed isn't the answer, what is? How about a slow shutter speed? Sure, we could use a 10 second exposure and if you could somehow pull the pan off, the background will be completely motion blurred, but how plausible is that?
Compromises
The trick is a balance. Sometimes a faster shutter speed might mean more keepers, which might be important to keep in mind if you're covering an event and it is the final round and you
need
a keeper. But then again, slower shutter speeds are more dramatic. With all this shutter speed talk, I think you've figured out what kind of camera mode you should be shooting in (Tv - Time Value Mode). In Tv mode, the camera will select the aperture for you based on the shutter speed you pick.
So, what's the ideal shutter speed? That's for you to figure out. 
Canon 20D | 135mm, 1/25s , f/14, ISO 100
Canon 20D | 85mm, 1/30s , f/16, ISO 100
Canon 20D | 100mm, 1/60s , f/14, ISO 100
Canon 20D | 85mm, 1/80s , f/11, ISO 100
Canon 20D | 85mm, 1/100s , f/5, ISO 100
Canon 20D | 85mm, 1/160s , f/4, ISO 100
Canon 20D | 200mm, 1/200s , f/13, ISO 100
Canon 20D | 135mm, 1/5000s
, f/2, ISO 100 (yes this is a joke 
)
TIP! The faster the car is moving, the faster shutter speed you are able to use because the car will be covering a greater amount of distance in a unit of time.
Nice article Dong.