Writing for the Atlantic, Marc Ambinder has up a solid and well-considered brief for anyone who's attempting to follow the news out of Iran on Twitter, rightly noting that the resource should largely be considered a source of "signals intelligence."
There's plenty of misinformation out there, like rumors that Ahmadinejad is going to stage an assassination attempt, so we need to be careful about how we judge the information. If we're a savvy analyst, we need to be careful about the weight we attach to photographs and video accounts. They're the most immediate and emotionally powerful, but they can distort our understanding of the situation, particularly of about the importance of specific developments.
Ambinder has a lot of good advice (and a shout-out to @persiankiwi, who's rapidly earning him/herself a reputation for reliable and comprehensive information), but I think this is the sagest bit:
Don't assume. Everyone assumes that Mousavi really won. But there is reason to think that the election was very close -- and that Ahmadinejad might have actually prevailed (although the evidence appears solid that his totals were significantly inflated.)
Not to be a downer, but that's pretty critical. One of the effects of following the various Twitter threads on this story is that you can get swept up in and borne aloft by the hopes of those who wish to see Ahmadinejad defeated. It's somewhat seductive. You can even detect a little of this in Ambinder's post, which he titles, "Follow The Developments In Iran Like A CIA Analyst." To borrow the title of Christopher Hedges book, it is a force that gives this community meaning. But in the end, what you want to be the truth may not be true, no matter how enthusiastically you wish for it.