Enough shoes and clothes to last you until you come back - they don't really carry American sizes. Even if you are rather skinny for an American you can have trouble fitting into Japanese sizes.
Otherwise it depends on where you're going... if you're going to somewhere outside of the Tokyo or Osaka areas where most everything non-Japanese can be purchased, you might want to bring reading material.
If you are unusually big, you might want to bring condoms. Japanese condoms are a little on the small side (but I think 90% of American guys will be fine with them).