Bona fide relationship must be made evident in order to avoid visa denial. Present as much documents as you can to prove the authenticity of your relationship.
Take with you numerous colored pictures shot during the period you've met each other. Have the photos dated. Original receipts of your fiance(e)'s transactions done in your country reflected by credit card statements, itineraries, boarding passes, travel tax receipts, terminal fee receipts, baggage claim checks, and airline tickets should come in handy.
Both you and your fiance(e) can write a new letter of intent for continuing engagement to demonstrate your ongoing relationship. Notarized letters from a couple of friends and family members are helpful, too. You may even include chat logs, e-mails, or snail mailed letters and greeting cards of your discourse about wedding plans, and any form of communication from the U.S. citizen's family reflecting their knowledge of your engagement.
If you have never been married, obtain a certificate of no marriage. Annulment decrees, divorce decrees, or death certificate of your spouse will serve as proof of being single.
You have to prove that your relationship is valid, legal, and true. Going back to the purpose of the K1 visa, you are required to marry within 90 days upon arriving in the U.S. The desire to marry must be reciprocal and deliberate with all prior marriages terminated legally.