Can you hit neutral and back into the same gear? Sounds kind of strange that it's only ona down shift.
The end result of double clutching is that the motor gets spun up to the same RPM (approximately) as the tranny so when you shift the gears mesh seamlessly and your syncros are unnecessary.Double clutching isn't a skill you really need until you drive a vehicle that either doesn't have syncros or the syncros are shot. There's nothing wrong with letting them do their job when they are working.
To double clutch:
1. Depress the clutch and move the shifter into a neutral position.
2. Still in neutral, release the clutch and rev the engine to approximately the right RPM for the gear you're moving into.
3. Before the revs drop shift into the gear you're moving to as you normally would.
When double clutching is done well passengers shouldn't be able to tell that your syncros aren't working.
Hope that helps. If you want more information on how your tranny works you can find it here: http://auto.howstuffworks.com/transmission.htm