I've heard that double clutching should only be used when your synchros are broken as it is slow and can be dangerous to your transmission if done wrong. This topic came up in another forum I'm in and this was the response:

Double Clutching

Double clutching is another technique that is a little easier on the tranny, but it takes too long. Since we have modern synchronized transmissions, we dont really need to double clutch. Unless, the synchros are messed up. You can tell.. if your gears "graunch" vigorously on the up shift, then most likely your synchros are messing up. To save $$$ down the road, Double Clutch it.

Before we get into the steps lets remind ourselves what synchronizers do. Sychro's are friction members that bring the layshaft & gear to the same speed before engaging. So, if your synchros are messed up, you'll have to make the shaft go the same speed of the new gear yourself. The bad thing to do, is to let your RPMs drop a lot while shifting. On the upshift, you give it a little gas before engagement. On the downshift... more gas.

1) Press in the clutch.
2) Take the selector out of 4thgear and leave it in neutral.
3) Let go of the clutch, and blip the right amount throttle while its in neutral.
4) Press in the clutch again, and then put it into the next gear.
5) Let go of the clutch...done!

Giving the right amount of throttle in neutral is the hardest part.
The reason for blipping the throttle while in neutral & clutch pedal is out: To bring the shaft up to speed. The shaft is not connected to the engine while the clutch pedal is in.
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1990 244DL - K&N, airbox mod, 2 15' JLs, Brickstylz body kit, underbody neon (2.5' straight pipe exhaust system, performace cam to come!!!) all at 255K+ miles. 1998 S70 GLT - mom and dad's car. If only....