So here's the 'bot guide to hosting:
1) Pick your date/time: If it's a broadcast, that's easy! Just let the networks schedule it for you. Otherwise pick a time that seems reasonable or, if you find yourself in agonies of indecision, set up a poll, suggesting a few dates/times.
If your account level won't let you have a poll, let me (warriorbot ) know by PM and I'll throw one up for you!
Then set up a countdown timer being sure to pick the right timezone! This isn't essential for broadcasts, but is darned useful for any "start your DVDs now!" type watchalongs
2) PIMP UNASHAMEDLY: About 5-7 days before, put up a "Coming X-day - YYY!Verse Watchalong" post on this comm with details of the arrangements (and the countdown timer). Tell all your friends. Work it subtly into your fic. Put the URL in a hidden code in your fan-art.
3) Get the party started: About an half an hour to an hour before the watchalong, throw up a post saying "Here's the watchalong for XXXX..." or similar. Encourage people to CAPSWHORE, Sparkle text, gifspam or - if you're some kind of sick, twisted freak, encourage them to make insightful commentary.
4) 'bot's final piece of advice: One thing I would STRONGLY suggest is that anyone who does host goes to notifications and turns OFF notifications for "Someone replies to my entry in a community." Unless you want 1,000 messages in your inbox o'course!
And PM me if you get stuck!
Love you all xx