The I-751 instructions give examples of the types of evidence you can submit to prove that your marriage was entered into in "good faith". The final example of evidence they give is this:
"affidavits sworn to or affirmed by at least two people who have known both of you since your conditional residence was granted and have personal knowledge of your marriage and relationship. (Such persons may be required to testify before an immigration officer as to the information contained in the affidavit.) The original affidavit must be submitted, and it must also contain the following information regarding the person making the affidavit: his or her full name or address; date and place of birth; relationship to you or your spouse, if any; and full information and complete details explaining how the person acquired his or her knowledge."
This is quite possibly the most complicated aspect of filing the I-751. I looked up the subject on the US Marriage Based Visas newsgroup. From reading several posts, this is what I learned:
1. It is not essential to include any affidavits.
I personally think it's a good idea to include at least two, as it should reduce the chances of you being called for another joint interview to prove the validity of your marriage. We did include 2 affidavits.
2. Some people take "sworn to or affirmed by" to mean that the person writing the affidavit should have it notarised. Other people believe that notarisation is unnecessary. Your affidavit writer could include this line instead: "I declare under penalty of perjury under the Law of the United States of America that the foregoing is true and correct. Signed on month day, year at my town, my state." If your affidavit writer is willing / able to have their letter notarised, then you may as well have it done. One of our two affidavits notarised.
We asked Ben's Mom and Ben's brother's girlfriend to write affidavits for us. I gave them a list of what had to be included:
Letter format:
Must include your full name, date of birth, address, and phone number.
Must be dated.
Must state when you met us and for how long you have known us.
Must state your relationship to us, if any.
Must state that you have seen us a certain amount of time [eg. “frequently” ] since we have been married and that we appear to be a genuine married couple.
(You can give examples of how we spent time together on specific holidays, birthdays & when we moved house.)
Must be signed & preferably notarized.
I then included example letters. Two were from Doc Steen's website. One was from the K-1 FAQ.
Here's what they came up with (Thank you):

*Full Name*
*Address*
*Phone Number*
July 28, 2003
To whom it may concern:
I am writing to verify that I have known Jennie ***** for a little over 3 years. Our son, Benjamin, invited Jennie to our home to meet us in June 2000. Jennie and Benjamin were married in August 2001. I am Benjamin’s mother, and we are delighted Jennie is a part of our family.
Since their marriage, we have seen them regularly at all family events and just to get together on week ends both at their home and at ours. They have worked hard together and have just recently been able to purchase their first home. We have all worked together on painting and repairs. They are a happily married couple, looking forward to their future. Jennie is a wonderful, and much loved addition to our family.
Sincerely,
*signature*
(Date of birth: xx-xx-xx)
(Birthplace: Town, State, U.S.A.)

*Full Name*
*Address*
*Phone Number*
8-31-03
To whom it may concern:
My date of birth is xx-xx-xx, and I was born in town, state. I have known Jennie xxxxx since March 2001. I first met her at a family dinner at a local restaurant. I attended her wedding to Benjamin xxxxx in August 2001. I see Jennie regularly at family events and holidays. I have spent the past three 4th of July celebrations with Jennie and Benjamin watching fireworks. I have helped them move into their first apartment in 2001 and recently into their new home in June 2003. They have worked hard together to make this home their home. They are happily married and looking forward to their years ahead.
Sincerely,
*Signature*

