Six months is not that long. I'd wait a little longer (say 3 or 4 months). If you act too soon, you are likely not to get much sympathy from the congressional aid charged with immigration issues. Sympathy is important (at least in my mind) - you want to get that person believing that your case is very important.
Both the congressmand and the senator that I contacted had "immigration issue" forms on their web site. Their process specified that I was to fill in the form (as well as and information release form), and then fax it to their office. In neither case did I send a "letter"
