I would say it's completely pointless for regular driving, just use the clutch pedal and the gearbox like any normal person would.

The way to preserve a transmission, would be to also regularly change its respective oil as well, to SPECIFICALLY the one recommended in the manual (most people don't do this with MANUAL transmission, only on AUTOMATIC ones which are more sensible and expensive to repair). This, because many car brands, can only have their own specific own car brand oil, and not a third-party one, even from recognized brands.

Next, when putting gears in, make sure the transmission isn't holding it back (like trying to put 1st while driving on the highway), potentially causing a grinding sound. If it does hold back, but the acceleration is decent, then I personally like placing it on neutral, and try again which always work.

I see some people putting in gears which such brute force, even when the gearbox is trying to hold it back. That is another way of ruining it.

It's all about not driving as a maniac, like very fast accelerations out of the sudden for no reason, or even doing burnouts which absolutely wears it faster than anything.

Additionally, after starting the car in a cold morning, let it idle for a few minutes to do that pre-heat up (where REVS usually go down from 1000's to 800's). Like the engine, the transmission also has its own oil (much ticker one) and moving parts that, which are most efficient when pre-heated, causing less tear and wear.

Lastly, going uphill from a stop, use the hand-break to assist you, instead of putting all the weight on the clutch (unless your car has the technology not to roll backwards automatically).

Check this out for more:

https://www.youtube.com/watch?v=_cbZlhduYJY

Double clutching is unnecessary with the synchros in use today.  The only thing double clutching will do is prematurely wear out the throw out bearing.  Remember, it's a truck, not a Porsche.  Give yourself a "one one thousand" count between shifts, up and down and make sure your clutch pedal push rod is properly adjusted to completely disengage the clutch when you shift.

Posted by: @neferati

to switch gears you have to press the foot on the clutch down for a temporary amount of time ...

Thank you. We are aware of how manual transmissions work. The difference between neutral position on your transmission, and using the clutch pedal, is that the clutch requires you to have your foot in it. In the absence of driver actions, the clutch returns to an engaged state.

You should always retain maximum control of your vehicle. Coasting leaves you unable to accelerate, and unable to quickly to respond to quickly changing road conditions. Also, if something were to happen to the driver, the car would be completely at the mercy of gravity and could cause unchecked damage or death to people and property. Keeping the transmission in gear also keeps the engine in a running state. It's much easier for an idling engine to stall in neutral, which would results in a loss of brake and steering assist, and possibly being unable to restart the engine.

This is all common sense. I highly suggest you find a driving manual and read it, or take a refresher knowledge test.

Posted by: @neferati

If the road conditions are good ...

This is a common excuse I hear all the time. The problem is people always overestimate their ability to react in emergency situations, and underestimate road conditions. A good driver establishes good driving habits which minimizes risk, prepares them for the worst eventualities, and develops the muscle memory needed to react in reflexive situations.

Everything you are divulging here tells me you have a naive and risky driving attitude.

For cars made since the 80's or 90's, I'm confident it doesn't really matter, as long as the transmission is well-maintained and nothing is wrong with it.

I know that on very old cars, changing the gears had to be not only sequentially (you couldn't skip gears), but also had to go through the neutral first. For a 2008 car which is your case? None of that matters anymore, don't worry about it.

I have owned many manual 90's european/japanese cars, and never needed to think on such things, and they always worked fine.

Although, what I personally do after driving hundreds of miles on gear and see there's a long straight downhill, I like to put in a neutral for a while to cool the transmission/clutch down. Yes, that is useful, and is detectable on older cars with iron engine block, where the thermostat gauge reacts to it by slightly decreasing.

It's also illegal to coast in neutral in a motor vehicle here in the U.S.  Check the law.  And @mmj is right.  It's extremely dangerous.

It is very easy to justify your points by predicating them with the word "if."  I don't know or care what country you live in but it is illegal to coast in neutral here and for very good reasons.  Have a wonderful evening.