Extra outfits in Dead or Alive are earned by winning the game with the character you want the extra outfit for at normal difficulty or higher.

You can earn the first few outfits for each character by setting the life bars for you at Highest and your opponent at Smallest and setting the fight count at one.

Once you have gotten all the outfits you can in this mode, the remainder can be gotten by setting the game to default settings (best of 3 rounds, normal life, normal difficulity, normal life bars) to get the rest. Using this trick you should be able to get Kasumi's first 10 outfits, Lei Feng's first 9 outfits, and everyone elses first 5.