Borrow a Life:
When playing a team multiplayer match, if you are defeated but your
teammate still has one or more lives in stock, you can use one of his
lives to get back in the action by pressing A+B+Z+START. Be sure
to ask for permission from your friend.
Change Outfits:
You can change your character's outfits by tapping the four C buttons
at the character select screen. Some costume changes are limited to
colours, but characters like Pikachu are also available with a hat.