The web, in general, is hardly a land of polite discourse and respect for one's fellow denizens. While Wikipedia's culture tends to do a good job of weeding out obvious trolls ("UR MAMA SO FAT"), users who are more subtle -- even sometimes quite productive -- but nevertheless problematic, can pose a significant difficulty.
Raul, I believe it was, had a lot to say on this subject at the Arbitration Committee panel of Wikimania 2006.
I've had a bit of a mixed bag, on this. I saw one Mediation Cabal case over linking to a school alumni group's website -- it had a wealth of source material, but the link was seen as promotional in nature (the main advocate of the link was involved with the group). After a week or two of tense negotiation, people had more or less agreed to link the site specifically as a source, and I thought things were going okay. Copyright concerns about the site were the last thing to sort through. And then a party in the mediation suddenly started trolling, accusing another party of anti-semitism, and all sorts of things -- out of the blue, as far as I could tell. I even remember asking them, "You're starting shit. You know you are. Why?" Guy never came back, and the whole agreement imploded.
So, that's one lesson: as a mediator, don't advocate. You're not there to see any person or any conclusion come out ahead, within the frame and goals of the project -- and if you are, you probably shouldn't be mediating. There's two important aspects, here: first, you have to be a neutral party, and second, people need to see you as as a neutral party. Unfair mediation should suck for obvious reasons, but if people don't think they'll get a fair "hearing," they won't bother to show up unless they have to. On a volunteer project, there's really no way you can force people into mediation (or anything, for that matter) unless they want to be there.
Then there was a time I responded to a request for third opinion, on whether a particular author qualified as being Jewish-Hungarian. Being an obvious outsider to the argument, I was actually perceived as a neutral party, and things were resolved quickly. Both parties accepted my opinion, by some strange miracle, since all they seemed to want was a tie-breaker. That was a good one.
I remember another case, also with MedCab, where somebody was offended that their article was deleted, and also hurt at the behavior of the users advocating deletion. I tried a sort of "fireside chat," talking things over with them for a bit. They only asked for an apology from one user, who -- once they became aware of the mediation request -- stormed in and continued the name-calling. Again, the newcomer left the project.
There's a few lessons I could draw, from that. One I usually try to look at is: encourage giving, get all sides in a dispute to make concessions, however small. This encourages people to play fairly, makes people see that everyone is committed to the mediation, and creates a feeling that progress is being made. Another lesson might be optimism: people need to believe that the mediation can work, and is working. If one person doesn't believe that, it's of the utmost importance to find out why, and fix the problem if at all possible. Users can almost always fix problems and arguments on their own; the trick is getting them to believe this, first.
Also be watchful for discussion. There are a lot of pages. People don't always run and grab the mediator(s) before arguing, so be proactive.
In a face-to-face mediation, people can't easily leave the table. In Wikipedia or any other online setting, doing so is not only easy but even sometimes difficult to notice. The biggest tip I've yet figured out is this, I suppose: get people at the table, both mentally and emotionally. Get them to vocalize this, encourage them to negotiate. Act as a middleman when they aren't willing to talk to each other.
And, of course, be careful with blocks, if you're an administrator. All too often, everybody in an argument wants everybody else blocked. If you ever accede to one of these requests, a lot of things can go wrong: (1) the blocked user feels wronged, and may not trust you to remain neutral as a mediator; (2) other users may see a chance to "win," and will probably badger you to block again and again; (3) while the user is blocked, circumstances may change dramatically -- be careful that their absense will help, rather than hinder, the mediation. Blocks can be necessary, to get someone's attention, to stop edit warring and attacks, but always be careful with them. We can afford to hurt a vandal's feelings, but good contributors are hard to come by. Be gentle.
Remember why you got into it -- with all the trials and tribulations, people yelling back and forth, and generally squabbling and refusing to work together until the most ridiculous concessions are made, it's important to keep your eye on the prize. Keep things together, keep the project together, and work for the best interests of the project.
This has been a bit of a ramble. More for my sake than a reader's, I suppose, just to get a few thoughts in order. But if you get anything out of it, all's well and good.