Look for PASSION. Resolving disputes is hard enough, you need someone who is committed to the process and passionate about their work.
There are many mediators of every type and with many different types of expertise. Think about your dispute and what you feel comfortable with. However, don’t forget at it’s base mediation is a process involving people and you should have someone who believes passionately in the process. Whilst you can daydream if you are doing something like hanging out the washing, mediators need to be committed and engaged.
Make sure that you speak with the mediator about how they approach mediation so that you can get a feeling for how they will handle your session. You may want a mediator with a particular style, range of experience or professional expertise.