Double clutching will make the synchronizers last a very long time, because by double clutching you reduce or eliminate the work they have to do.
An easy way to tell if you are double clutching properly, is to watch the tachometer. If when you finally complete the shift and let out the clutch the engine RPM should not fluctuate any. In other words you want Exactly the same RPM just before and after you release the clutch with the trans finally in the chosen gear.
The goal is to use the throttle to set the engine RPM to exactly what it needs to be for the road speed and gear (double clutching as needed so the transmission is thinking the same thing), then let the clutch out and continue driving.
Grigg
