I had to file I-751 after been divorced. Get as much documents ( bank statements, tax returns,car and health insurance, pictures ,etc) anything that have both your names on it. I have submitted the copies of above mentioned documents with my I-751 form. You will have to proove that your marriage was not to obtain immigration benefits. Try to find an immigration center that may rpovide you with an advice regarding supporting documents, etc. I found one in Akron, OH . Consultation fee was $ 50. I believe you can get addresses through the Internet. I also hired a lawyer that was very helpfull. When I was searching for a lawyer, price varied from $ 500 to $3000. Good luck!