Ubertricks are pretty difficult at first, but stick with it and eventually you'll start getting those ubertricks! Here are some tips to get you going:
Get Some Air!
The more air you get off a jump, the more time you have to perform an ubertrick. So, don't attempt ubertricks on small jumps, you'll only frustrate yourself.
Practice On The Half-Pipe
The half-pipe is a great place to get big air easily. Once you start feeling comfortable controlling your character on the half-pipe, you will start flying through the air and have plenty of time to practice ubertricks.
Draw Those Patterns Slowly
Make very deliberate motions when drawing your patterns. Most people tend to draw the patterns very quickly, since the gameplay is so fast, but this rarely works. If you are having trouble performing an ubertrick, trying slowing your motions down.
Practice in the Pause Menu
Use Your TV Screen As Pattern Reference
The "Z" pattern trick is the easiest trick to perform. After you jump, hold , point the wiimote to the top left corner of your TV screen, motion to the top right corner of the screen, to the bottom left of the screen, then to the bottom right of the screen to make the "z" pattern, then release . Pretend that you are drawing the pattern inside the edges of your TV screen.
These pointers should get you started on making ubertricks, but as usual, practice makes perfect.