It's important to understand the pace at which you need to press the buttons. There is a particular speed you need to tap A to get the wobble right.
If you are not of the musically inclined, I suggest using a metronome. You can download one off of the Google play store or App store for free. Set the metronome to 200 bpm (beats per minute) and happy wobbling.
Another issue I've seen people have is the fact that they don't desync Nana and Popo or they don't keep Nana in a tilt attack.
Nana needs to be performing a Forward tilt or a Down tilt while Popo pummels. That means you need to keep the control stick down or in the direction of the opponent the entire time the wobble is being performed. (I learned with the Down tilt because it is easier to keep going once you get it. It is much harder to start up though and I have since switched to the Forward tilt).
If you need help with desyncing the Eskimos I believe there is a thread here with that information and there are plenty of YouTube videos showing you the three basic desyncs.
I hope this helped you man, and if it did... Happy Wobbling!