A number of people have asked me what I used for proof of relationship for the K1 visa application, so I thought that I would post up what I used. In our case is was similar to the I-129 petition relationship proof that we used, when my wife put in the I-129 application (although we probably gave the USCIS more than they needed at that stage).
Basically from I learnt, the standard proof of ongoing relationship that they would accept were things like photos of my wife and I together and together with family, postmarked letters and emails that we had exchanged, telephone bills that showed we had spoken over the phone, and flight tickets that showed we had visited one another. You are allowed to censor out some of the more personal information from letters etc. I believe.
Anyway, the specific evidence that my wife and I used as K1 proof of relationship was:
Photos of us during a visit. There was a photo of the two of us together, a photo with the two of us with my fiancee’s daughter, and a photo of my fiancé with me and my family.
The plane tickets that were used for the flights across the Atlantic, and also the ferry tickets that we used on a day trip to France together.
Some of the cards that we had sent to each other by mail – birthday and postcards.
Some printouts of emails that we’d sent each other. Nothing too personal, of course.
Some printouts of MSN messenger and Skype conversations that we’d had together.
A phone bill to show that we had telephoned each other. (Although that didn’t happen more than a few times as we generally used Skype to communicate, basically because Skype was cheaper and also we could use webcams, which is more personal).
I mailed my proof of ongoing relationship to the London Embassy, which was the correct procedure at the time, but I think someone told me that visa applicants now have to take their proof in by hand when they attend their K1 Fiance Visa interview. The rules and guidelines can change at any time, so always double-check with the USCIS website to make sure that you are doing the right thing beforehand.