double clutching:
you're in first gear, ready to go to second (or vice versa)
Clutch in, start shifting. As the shifter passes through neutral, clutch out, then blip the throttle to match the engine revs to the revs it will be doing in whatever gear you're going to, clutch back in, put it in gear, and clutch back out.
The only clutch component you're wearing on is the spring, which generally lasts a LOT longer than the friction material anyway. Because you're rev matching, you're not really wearing out the friction material because it's not spinning against something that's going a different speed.
If you do this correctly then the only place you're really causing any wear at all on the transmission/clutch is when you're starting out in 1st gear, since you have to slip the clutch a bit there.
btw NG, you can downshift to 2nd at 45 and still be smooth as silk, if you properly match the engine revs to the tranny revs.
In fact, if you wanna be technical about it, if you get good enough at rev matching you can shift without the clutch at all. If the engine is going at EXACTLY the RPM it will be going in the new gear, the shifter will slip right in without needing the clutch. It's a handy trick to know, especially when the clutch cable decides to break on you 50 miles from the middle of nowhere.
A lot of semi drivers do this to save wear on the clutch - that's why when they shift gears as they're getting going you hear a long pause between gears - they're waiting for the engine RPM to drop to that of the new gear.
Note that when you're learning how to do this you're going to crunch the gears a LOT, so do it on a car you don't care about
