Last Halloween I showed up at a nightclub in Shibuya wearing a pair of fangs and a sexy dress and expected to get the costume discount. The doorman laughed in my face.
The Japanese don't mess around when it comes to costumes. Last year, the ghouls in Shibuya stepped up their game and I saw some pretty incredible outfits and makeup — one girl looked as if her face had been ripped off (I might have had nightmares from that one). In the land that created cosplay (dressing up as your favorite anime or manga character), cutting two holes in a sheet and saying you're a ghost isn't going to cut it.
Having said that, it isn't hard to win at Halloween in Japan. First thing I'd suggest is give yourself some time to think about who — or what — you want to be. One trend that kept coming up last year was groups of friends who dress alike or according to one theme. I saw the bloody nurses of the "Silent Hill" game series, Disney princesses and small hordes of salaryman zombies. So if your friends are up for it, go out together, you're more likely to get noticed.