As you noticed the trick is staying right on top of the opponent, so you have to get used to the jump angle used when you first jump at them from the ground. I found working on the C combos after the 4B launcher was a good start, as the jump is just straight up. Just mess around with that until you can get the C juggle down pat, then work on picking up people off the drive combos.
*shrug* I also found real matches were a good way to practice the dj.C portions of the combo, as you tend to hit people in weird locations when they're trying to kill you. After that, regular juggles are cake.