There are a number of courtesies that drivers extend to one another...
Introduction
Unfortunately, some of them are slowly dying… maybe they're not being taught to new drivers, maybe life is simply becoming more impersonal, it's hard to say. None the less, extending courtesy towards your fellow driver is a display of professionalism.
Waving on 2 lanes
Drivers often wave at one another on two lane roads. Years ago, before the CB radio became so popular and you had to have a Ham license, drivers had hand signals they used to communicate different messages, such as: accident ahead, speed trap ahead, etc. Though the hand signals are long gone, waving is not. It is a nice way to acknowledge a driver.
Flashing headlights off and back on after passing
When a truck is passing another truck, quite often the truck being passed will flash the headlights off and back on to "tell" the passing truck that he/she has cleared him, and can bring his/her truck back into the right lane. This is done more often at night but not necessarily only at night. Some drivers flash their high beams instead and this serves basically the same purpose but many drivers do not like this because it can be blinding. Some trucks, though, have headlights that cannot be turned off so there's really no other way. This courtesy is not really done because the driver doing the passing doesn't know when he's cleared the other truck - it's done as a courtesy - as a way to acknowledge another driver. Some drivers prefer to say something on the CB radio instead, such as, "Okay driver, you got me." Beware! Many states say that this practice is illegal! For that matter, any gesturing to another driver, as in a hand wave for someone to "come around" can make you liable in the case an accident results. Most drivers still wave/flash for other truck drivers, but do not and will not for cars.
Saying "Thank You"... please mind your manners.
After being "flashed" over by another truck, it is only good manners to say thank you! This is done by flashing the trailer lights. You can do so by flipping the switch on the dash or by putting your four ways on and letting them blink 2 or 3 times. You can even pick up the CB mic and say it to the person. You can say "thank you" in this fashion in other situations, also – when another truck stayed back so you could complete a turn, etc.
Stopping Short or "Hanging Back"
Drivers are always "looking ahead" to what they're driving into. When a driver can see he/she is going to arrive at a turn at the same time as another truck, typically one of the trucks will "stop short" or "hang back" so that the other can make the turn. Since trucks need a great deal more space when completing a turn, this makes the whole process more efficient. Since many turns require the truck to take most, if not all, of the available space, if two trucks were both pulled all the way up to the turn, neither would be able to go.
Turn signals
Drivers are usually very good about using their turn signals. This is yet another way that a professional driver stands out from the rest. Four way flashers are often used to communicate to others that a truck is moving very slowly, when parked along the side of the road or when backing. Drivers are usually very good about using the proper signals, turning or 4 ways, when needed. Again, it is a sign of professionalism. When backing out of a truckstop spot that is between two other trucks and in effect, completely blind, drivers will turn on the 4 ways and the CB radio and proceed slowly. If there is someone coming or some other obstacle, most the time someone will say something on the CB radio to you. Be sure you're also watching for other trucks' four way flashers when driving through the truckstop lot because it is usually going to mean that that truck is preparing to back out of his/her spot. As a courtesy, stop and allow him/her to get out of the spot. By you sitting there waiting, you'll also be helping block the space.
Not Tying Up Left Lane
The left lane of traffic is supposed to be the "passing lane" and the right lane is the "traveling lane". Some states have enacted laws to this effect and have posted signs that say something like, "Stay right unless passing". Since a truck is usually moving slower than other traffic, it only makes sense to stay out of the faster traffic. It is also very important to guard the blind area beside the passenger door on the right side of a truck. Preventing others from passing by blocking the left lane makes people angry and they will only wait so long before doing crazy things to get around - such as passing on a shoulder, etc.
No Headlights in Truck Stop at Night Unless...
a truck stop parking lot is very dark, most drivers turn their headlights off when entering. If you are parked in the lot, sitting in the driver's seat in your truck, putting your four ways on and letting them blink 2 or 3 times. You can even pick up the CB mic and say it to the person. You can say thank you in this fashion in other situations, also – when another truck stayed back so you could complete a turn, etc.
No Headlights Entering a DOT Scale
Even the DOT typically does not like it when you enter the weigh station with the headlights on. Some post signs that tell you to turn them off. From the place they're sitting, your headlights blind them.
No Horn at Truck Stop
Many students are taught at truck driving school to honk the horn just before backing up. While a good safety precaution at a driving school, using this technique at a truck stop is terrible etiquette! Many drivers are sleeping and this isn't the way most like to be awakened.
Don't "Park" in the Fuel Line
Many drivers will pull through the fuel island to run inside the truck stop and get coffee, use the restroom, etc. While this doesn't usually create a problem, some drivers will literally park there for a long enough time that someone pulls in behind them, fuels and then ends up being blocked in because the other driver is in the truckstop taking a shower or sitting down for a meal! This author does actually endorse and recommend that women drivers pull through the fuel line at night to go in and take care of business but ONLY if it's going to be short and sweet. To keep another driver waiting because you're too lazy to actually park in a spot is terrible etiquette. Sometimes there won't be any available spots and you'll be all but forced to do this - but please, make it quick.