Plan your purchases. Get the handbook and see what convention limited stuff you'd wanna get and line up for them. And get in line early to enter Big Sight. I lined up over night (crazy, hot, smelly experience) and it wasn't pretty....if you're not as fanatical, try to get onto the 1st train in the morning. If you don't care about those limited ed stuff either, just screw it and come in the afternoon. At least there won't be a queue entering the venue.
Wonfes is huge, when i went all 3 halls were open. The industry hall, the garage kit hall, and the seller's hall. You might find some bargains at the seller's hall. The garage kits hall is huge and will take half a day to look through if you like garage kits. The industry hall is where the big players like Kotobukiya, Yamato, Bandai etc will setup their booths showcasing latest products or selling convention exclusives. The queues to get exclusives can be pretty long though so be prepared to do alot of queueing up if you want to get multiple items. Its not nearly as chaotic as Comifest (which i swear i will never attend) so I think you'll be fine if you're just a casual visitor.
EDIT: Now i feel stupid.....this is an american con, not the Japanese one. =.=;;