After you win a match and your character does his/her victory pose, press Start. You'll be able to attack your fallen opponent while he/she is down.
Enter it in at the versus screen after you select your characters, and hold it until the match begins
Unlock all the costumes and hidden characters to be able to choose the same character mutiple times on the same team.
Set the game in Training Mode and let it sit--every minute you let the game sit will earn you more points used for unlocking characters and costumes. You can earn up to 9,999 points at a time doing this.
To get the hidden characters at lower prices, keep on entering and exiting the shop. Eventually, the hidden characters will cost less.