• Do a manual while riding close to a wall.
  
• Press X and hold triangle and either □ or O, and tap a directional button to perform a trick.
  
• Keep holding either □ or O and you'll see your skater doing a trick while wallriding.
  
• Tap ↑ ↑ x, ↑ ↑ x, as many times as you can while still holding either □ or O.
  
• Release the □ or O button and you should fly up and get a ridiculous amount of air.
  

Note - It is also possible to do this without manualing first, but it is a lot harder.