Avatar ad on the NXE states that new fashions will be added every two weeks and will be noted by a gold star. Xbox Live Product Unit Manager Jerry Johnson confirms that new accessories will be added for at least the next half a year.
According to Johnson, the new fashions will be implemented in packs. We've already noticed that some clothing options have been taken away, like the "wife-beater" (no, it wasn't called that on the NXE). No word yet if these packs will cost anything, but we do know that Microsoft is considering it. What, you don't think the virtual Louis Vuitton bag or Marc Jacobs outfit (not the game designer) will be free, do you?
Source - Avatar theme packs confirmed for next six months [VG247]
Source - Avatar fashion selection to double in next six months [X3F]