Flemming, Jyri is right! Also, mind your right elbow. It tends to flop out of the vertical plane, both going down and coming up again. Try to move more from the shoulder.... Think of your shoulder as the pivot of an imaginary wheel that extends right down to your hand, and on through the whip - a wheel that's absolutely flat. That wheel should move flush against the imaginary 'clock face' Robby talks about.
Another hint from Robby: keep your thumb pointed in the direction you want the whip to travel. That way, you won't rotate your arm as it move, which can cause the whip to 'spiral'.
As Robby has said so often, take it slow! It doesn't matter about the crack. Get the form right, and the crack will come. (My music teacher used to say, 'If it takes you five minutes to move from one note and the next, that's fine: what's important is to practise accurately. Otherwise, you just learn your mistakes! I think the same is true with whip-cracking.)