If you're represented by counsel, meet with him/her in advance to discuss the goals you hope to achieve in mediation.
If you are not represented, bring recent medical records, objective medical tests, initial accident reports, any State Board of Workers' Compensation forms you've received, and any other important reports or records you want your mediator to review.
Dress comfortably (but not slovenly) and approach your mediation with patience and calamity.
Mediation is a voluntary, non-adversarial process and should not be dreaded or feared! You can speak as much or as little as you would like to, and there is no cross-examination.