This is an easy thing to do. I found that when I do it, I keep tension on the rope and begin the ollie before I completely straighten out. If you wait till you are going straight, then the tension will be removed from the rope and it will just be that much harder. I good way to learn it is to use the wake to help you at first until you get used to it.
Ride up the wake like you would for a butter slide, only instead of sliding, ollie and then turn the board. If you land it short, you can recover easier on the peak of the wake. The other thing I discovered, is not to think about it too much, just do it. I didn't plan on doing it the first time I did, I just got out there and everything seemed right so I did it. Landed it the first time I tried it, and every time since. Had a few sloppy ones, but never failed to complete it.
Good luck to you!!