It seems that many folks here can't seem to read or understand basic english. The question posed was extending the life of the transmission, not the CLUTCH. there is a big difference.
There is a lot of disinformation out there, but even on the net, there is very little information on how one can extend transmission life, and by extension, the proper correct, and best way to shift a manual transmission.
The gearsets spin at different speeds. Modern transmissions use synchronizers or "synchros" to brake the faster gearset so that it is rotating at the same speed as the slower gearset so that the shift can be made. Older gearboxes without synchros, or if your synchro is worn out, need to have the RPM precisely matched in order to allow the transmission to shift from one gear to the next.
Understanding that a "synchro" is simply a small brake pad (or clutch pad) inside your transmission sheds light on how to extend the life.
a) gentle use of the brakes, slow and gradual application, extends brake life. by extension, slow and gentle shifting, even when driving aggressively, reduces the rate of wear of the synchro
b) matching RPM when upshifting, by pausing long enough for the RPM to fall to as close as it will be for the next gear, minimizes synchro wear when upshifting. This should be mastered first as it doesn't require throttle application.
c) next, matching RPM with a blip or stab of the throttle (gas) when downshifting. For starters, most people downshift excessively and unnecessarily. If you are braking to a stop, use the brakes, as that's what they are for, and brakes are far cheaper than transmissions.
If gearing down from 4th to 3rd, engine RPM will tend to rise approx 4/3, or 30-35%. understanding some basic math, a blip of the throttle to raise RPM from 3000 to 4000 RPM will essentially match RPM in the lower gear. This takes practice.
Racers use a technique called "heel and toe" which takes tremendous practice and understanding to master. This allows matching RPM while continuously braking, and is beyond the needs or skill levels of 99% of drivers out there.
d) shift less. this sounds pretty basic, but don't shift if you don't need to, as every shift creates wear on the clutch and transmission.
Shift less, use your brakes to slow down, and skip gears when possible.
e) skip gears when upshifting. Most cars have close-ratio gearboxes, and motors with broad torque curves. Its entirely possible to skip one gear, and sometimes even two.
I commonly make use of 1-2-4-5, 1-2-3-5, 1-2-4-6.
f) if your car has short gearing, or you are pointed downhill, consider making use of a 2nd gear start. This makes a lot of sense as 2nd and 3rd gear synchros are typically the first to wear out. starting in 2nd gear will reduce synchro wear on 2nd.
g) if a synchro on a specific gear is getting weak, skip that gear whenever possible
h) hold the gearshift with three fingers (thumb and index and middle) or wrapped between the 4th and pinky. You will apply less pressure, you can still shift quickly, but it will be more gentle and you will feel the engagement of the synchros better
i) don't use your fist to hold the gearshift lever, as you lose all sense of touch of what is going on inside the gearbox
j) don't shift like you are on the dragstrip. rapid-fire shifting saves you 0.05 seconds on your shift but, just like slamming on the brakes, creates a huge amount of wear on the synchros.
k) keeping RPM low. synchro wear comes from the differential between the speed of the engine, and the true speed of the car in that gear (which is the speed at which the gears are turning). Using higher gears to keep the RPM lower, at any given speed, reduces fuel use, but more importantly, minimizes the RPM difference between the gear you are coming out of, and the gear you are going into.
Driving manual transmission is very quickly becoming a lost art. I'm surprised there is so little information online about how to properly drive a car with manual transmission.
Most people have no clue. Think of your transmission as having a brake pad on each gear, which is really what a synchro really is. All the lessons that apply to extending brake pad/rotor life, apply equally to synchros.
Use the transmission less. Use it gently. Understand that it is a wear item, every time you shift. Slam on them if you must but understand you are abusing it if you do, and reducing its life.