Look forward and not back: try and focus on the future, rather than dwelling on recriminations for what has happened in the past;
Be clear about the issues you want to address: think carefully about the matters that you want to discuss and about your priorities;
Be willing to listen: this is not always easy when a relationship breaks down, but if you are able to listen to what your x partner is saying you are more likely to be able to reach arrangements that meet both of your requirements;
Be willing to compromise to achieve a fair and acceptable outcome: be willing to explore with the mediator all of the various options for resolving the issues to reach the one that best suits you and your x partner;