I practice DBL CLtchng all the time in the crossfire, it's not that difficult to do one you learn the timing on the throttle lag, it's much easier to do with the sprint booster but those are overpriced for what they do and cuase poor throttle resolution. I actually don't touch the clutch pedle during dbble cltching, what you need to practice is how to unload the tranny with the gas pedal, and then I manualy pull the shifter out of gear, then blip gas and shift into lower gears. if your wasting time pressing the clutch twice your going slower than the other guy and you will cause excessive wear to your throw out bearing and clutch..

down shifting I always blip the throttle to ease the loads on the syncros, remember they are usually softer brass, maybe some plastic parts too so the less work you make them do to keep the internal gears at he correct speeds for shifting the longer your tranny will last.