Keep it Short
The best affidavits are short, about 1-2 pages in length, but not too short. So, start out with the idea that you are going to write about 3-5 paragraphs.
Cover the Basics
Start with a short paragraph addressing how you met, how long you have been together, and your plans for the future. Address your last in person meeting (which must be within the last 2 years) and the length of time you have spent together. (i.e. On my last trip, in December 2008, we spent three weeks traveling the British countryside.)
If you have any of the red flags for visa denials (age difference over 10 years, language barriers, etc.) to overcome, you may want to address them in your affidavit. When it comes to these issues you are smart to seek the advice of an immigration attorney experienced in handling difficult cases. However, if you have to go it alone, you may want to make a negative into a positive. For instance, if you have a large age difference you may want to talk about how your common goals, interests and hobbies overcome any barrier created by your age. For example, "Even though I am ten years older than Karen, we have so many things in common. We both love running marathons, raw food, and healthy living. It was wonderful to find someone who shared such a commitment to green living."
Oath of Intent to Marry
One of the most basic requirements for the K-1 visa is that you intend to marry within 90 days of your fiance entering the country. Conclude your affidavit with a statement such as "We intend to marry within 90 days of my fiance's entry into the country."