Make Certain Both Sides Are "Ready" to Mediate
Mediation is neither a "quick fix" nor a painless alternative. It requires careful preparation, patience, openness and hard work. If both sides are committed to the process, mediation will succeed, but it won't be easy. By definition, each party will leave with less than what they felt they truly deserved. The benefits of helping to fashion your own solution easily outweigh the costs, but there are still costs which must be assessed going-in, or the process will fail.
Decide Whether Lawyers Will Attend
A lawyer-attended mediation has a significantly higher chance of success. Without legal representation present, the "lone mediator" has too many hats to wear. "Lone mediators" often require multiple sessions. The parties are usually in the same room. These features aren't necessarily bad, but a lawyer-attended mediation (where the mediator shuttles between the rooms) provides a dramatically different experience than when two parties sit down with a single mediator. The per-day cost is higher but the chance of reaching a complete resolution in one session increases many-fold.
Select the Mediator
The personal chemistry between the mediator and the parties (or between the mediator and the lawyer/client combo in each room) is a key factor in reaching a successful settlement. Good mediating requires a highly skilled mediator - well versed in the law, but also in the dynamics of negotiating in a high-conflict environment. For a lawyer-attended mediation, the two attorneys should select the mediator. When using a "lone-mediator" model, seek a referral from your attorney or call around to various family law attorneys.
Prepare, Prepare, Prepare
Don't just show up and expect a miracle to happen. Condense your case and its most important issues in a concise brief or cover-letter for the mediator. Carefully choose and organize your exhibits. Know the weaknesses of your own case, not just your opponent's, going in. Come with an open mind so that you can spend at least 20% of your time with the mediator, "living in your opponent's shoes." If the mediator can't get each side to at least consider the conflict from the opposing point of view, then the chances for success will drop.