Clean Pause
When you are playing, press START, then hold X button. (Try, with controller one and controller two, press START button quickly and many times during the game; you'll see a lot of letters in the screen.)

Level Select
In the title screen, press and hold X+Y+Z+START. Now you can choose the "SP battle" mode in the menu screen and you have will all the characters clear.

Disappearing Trick
For disappear, wait for the moment that your opponent will attack you projecting you down or up. Until that moment, hold A+Up, and your character will give a disappear and will give a punch or a kick in your opponent.

Note: this trick works better with weak enemies such as Kuririn and using fast characters like Goku, Vegeta or Picollo