problem for me is the apple homepage is preset as my browser home page.
My routine:
a) set iPhone to Flight Mode (starting with iOS6 this is not necessary anymore

)
b) Disconnect all messenger programs such as Adium and Skype
c) (in case you get spoilery mails, I don't

Close Mail/Sparrow/what have you
d) set Safari homepage to
https://www.macrumors.com/2012/09/11/apple-september-12th-media-event-spoiler-free-video-stream/
or
http://insightview.com/keynotechecker/
e) "New Tab opens" BLANK WINDOW!
f) Never open "Top Sites"... big banners on websites are often big enough to spoil in Top Sites
g) Check Apple's 1080p Keynotes Podcast all the time for updates (STEER CLEAR OF THE iTUNES STORE PAGE! Subscribe to that podcast beforehand!)
h) Don't use Twitter/Facebook/whatever
i) Steer clear of news sites (general) - Apple news, even during the event are HOT STUFF for them, they'll spill it as soon as they can!
j) Generally, limit your browsing only to the most spoiler-free sites imaginable.
k) Paranoia maybe, but definitely a thought to include: Adblock needs to run!
l) Watch some TV shows in iTunes (not on TV, TV stays off. TV is bad for you, mkay?) TV Shows let you interrupt them to do your checking routines, whilst movies just feel less exciting if you constantly pause them.
That's what I do.
(most of it)
Should keep you pretty spoiler-free.
Damn, I miss those live feeds from Apple.
Bring them back, ffs!
Glassed Silver:mac
edit: / addition:
Tips:
1. Watch videos on Youtube, preferably old Apple keynotes to get in the mood.

2. Start a conversation in this thread.
Are you crazy? Steer clear of Youtube! :O
Download them beforehand if you wish, but boy, you don't want to touch Youtube with a 5' pole!