Company has not reimbursed expenses or paid commissions

I was an employee working for an Illinios based company on a commissioned sales basis. This firm was to provide me with per-diem and mileage to offset travel related expenses that I paid for out of pocket. During a two week period prior to leaving the company I incurred $1400.00 in hotel and rental cars expenses. Pursuant to company policy I submitted a voucher detailing my travel, per-diem and mileage but never received a check for reimbursement. Furthermore this firm owes me commission on a project that I sold. I have made calls and faxed a letter to payroll to no avail. Now I find out that others who left the firm are also experiencing this same treatment. As well I also discovered that former employees of the firm had to file a complaint with the Illinois Attorney General to get paid back commissions and expenses due to them. What can I do?