So you are lucky enough to have recognized, and have a spouse/ex-spouse or partner that has recognized, mediation as the best way to resolve your conflict. Whether your mediator is in Chicago or suburban Cook County, or DuPage or elsewhere in Illinois, you will want to come to mediation prepared.
If you are mediating financial issues in a pre-decree case (so prior to divorce being finalized), you should make a list of your income and property. The income should list all sources of income and approximately how much you earn annually. If you only have one, salaried position, this should be simple enough. Have back-up documentation, such as your last year's income tax return with forms W-2 and 1099, and a recent pay-stub with year-to-date income.
For property, list all your assets, along with values. For example, you would list your residence, retirement account, bank account etc. The list should also show all your debts. You may want to list loans against property along with the property. For example, the house worth $250,000, with a loan of $200,000, leaving equity of $50,000. List all credit card debt and student loans, etc.
If you are mediating children's issues, have your children listed by name and date of birth. Write out your issues, the ones you wish to resolve. For example, parenting time, if you want more. Then write what you think is the best outcome you hope for and what is an acceptable outcome (what you would consider a compromise).
The complexity of the issues will determine what needs to be prepared. If the argument is over custody, you should list all the reasons why you having the child primarily reside with you is in the child's best interest. Regardless of the issue, if you were to lose, what would be the next best thing for the child?
Many mediators will have their own questionnaires or lists of what you should bring. At the very least, preparation in advance will help you identify, narrow the issues and make a more succinct argument.
For more information about mediation, feel free to contact me.