Write a list of grievances. If you are mediating a dispute between your roommates, have every roommate involved take the time to write down their grievances and feelings about the situation before they actually face each other. In addition to ensuring that everyone gets to relate all the details they feel are important to address, this process could also cause your roommates to see the triviality of their issues with each other as they are writing them down.
Meet in a neutral location. Since you and your roommates will all have to live in the same close quarters after the dispute has been mediated, hold the mediation itself somewhere else. This will help avoid anyone feeling that they are being ostracized or criticized, it will put everyone on equal ground and it will also help keep any hostility between roommates from lingering in your home.
Give everyone the opportunity to speak. Even if the dispute only directly involves two roommates, chances are good that everyone living with them has been affected by the conflict. As a way of letting all the roommates address their feelings and issues, give every roommate the chance to participate in the mediation even if they did not start it.
Work it out. Because roommates live in such close quarters, a dispute that is not completely resolved will only get worse and will cause the roommates involved to harbor resentment towards one another. To keep the house from becoming a negative environment, continue the mediation until a resolution has been reached. Obviously, it's possible that not everyone will be completely satisfied with the outcome, but as long as the roommates feel that they have been heard and that an attempt was made to rectify the situation, the outcome should be positive.
Remain neutral. A mediator needs to remain neutral in order to successfully carry out mediation and to be fair to all parties involved. If you are close with any of the roommates directly involved in the dispute or you have strong feelings about the issue being debated, you probably aren't the best person to act as mediator. Make sure that you can remain open-minded and objective for the duration of the mediation.