In the past few months I've come up with some of my own guidelines that I use when metamoderating. I use these to metamoderate quickly and (I believe) fairly, without having to spend large amounts of time reading articles and comments that may not interest me in order to contribute to the community. So, here they are, available for your use, approval, disapproval, comment, etc:
1) Always metamod positive moderations fair.
While I may disagree with the moderator about whether or not a post really deserved a mod up, this is taken care of by the moderation system itself. Posts that are modded up are seen by more moderators, so if they don't deserve it they're modded down quickly. I believe there has been only one occaision on which I metamoderated a positive moderation unfair, and the post in question was a patantly obscene flame against the author of the post that was being replied to.
2) Scrutinize negative moderations carefully: Mods down hurt posts. Once a post reaches 0 or -1, or even 1 or 2 if it's a big discussion (eg anything on the front page), it becomes lost in the soup and easily glossed over--even by people who are moderating. Most negative moderations that I encounter are fair, but it's not unusual for me to metamod one or two as unfair in a week.
3) When in doubt, look at the post in situ. I know /. discourages this, but here's my reasoning: If I'm not sure whether or not a post really deserved it's -1: Troll in the context of a deep discussion and several nests of comments in which it may well have been a hilarious, if subtle joke, an easy way to tell is to look at the final moderation. If it ended up as +5: funny, chances are it's not a troll.
4) Leave Redundant moderations alone. If the moderator thought it was redundant, it probably was, but I don't want to engage in the exhaustive searching that would be necessary to confirm or deny the point.