There is no standard or recommended format. At the congress rep website (*.house.gov), find your Representative and go to their site where they will have instructions to contact them and usually a release form. Senators have similar sites/forms. Most have specific staff members who specialize in dealing with BCIS.
In order to file an 'expedited processing' request, there are different criterion:
Significant loss of revenue by the petitioner
Compelling National or Regional Interest
Significant job creation or loss
and a few others i dont remember.
If your I-485 delay causes any of these (or your case falls under one of these and INS delay will result in job loss/revenue loss/etc etc, the congress rep office can help by making queries about your case and filing letters in support of the expedite request.
check with your lawyer. they will know more.
