If you are filing immigration petitions/applications without an attorney, there is one crucial mistake that most people make. That is that they don't keep a copy of the application they send in. This includes a copy of all the supporting documents that you send in.
The USCIS (or any other governing body) is a huge bureaucracy, and often lose or misplace petitions/applications/documents that are sent to them. Sometimes, they will claim that you did not send them a particular document and subsequently delay or deny your application. This delay can be create a great deal of hardship to the petitioner. Trying to get a copy of your application/petition/documents back from USCIS can often be frustrating and sometimes impossible. For example, if USCIS asks for additional information, they will only give you 30 days to respond. The likelihood that you can request and receive a copy of your original application within the 30 days, and allow you time to craft an appropriate response, is slim. While an extension of time is possible, it is not guaranteed.
When USCIS sends the petitioner a request for [additional] documents, the petitioner is often confused about what has been previously submitted, or if the USCIS asks for a document that has already been submitted, the petitioner has to go through to the effort/time/expenses of "re-obtaining" the documents, since no copy was kept. Further, often the USCIS will question a petitioner's answers to a petition/application. Without a copy of the petition/application submitted, most petitioners have forgotten their answers. Further, having a copy of the petition/application will also help an attorney understand your case and provide a record for an appeal, if your petition/application is later denied.
Important things to have a copy of:
1. your application/petition
2. any supporting documents sent in with your application/petition
3. copies of certified mail return receipts (you should always use certified mail or some other traceable method, so that you have proof of WHEN your petition/application was sent in, in case the USCIS questions your petition/application for timeliness).
4. all letters/corespondences between you and USCIS.
James C. Tai