It would be really handy if you could post some videeo footage of your delivery, that way we could make a judgement on how you are throwing the ball, and give tips on where to improve.
some general tips though...
try to keep your wrist cocked and strong through the delivery, and your hand behind the ball on release, with your fingers pointed at 10 o'clock (in other words, point your fingers at the pindeck of the lane immediately to your left (for a right hander) and oppisite for lefty's.
I also find it helps to tilt the ball slightly outwards (away from your body), and it helps generate more sideroll, which is what gives that arcing backend reaction.
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