One size does not fit all and you must find your own way. However, I can tell you about my experiences.
Some people like to talk all about themselves. I started doing this when I came back to snail mail in 2008. I didn't receive many replies to those intro letters. A huge disappointment. Then, I thought about it - the all about me me me me letters could be selfish and one sided.
Letters should be ongoing conversations. Questions should be asked but not too many - you do not want it to be an interrogation or inquisition.
I changed the way I approached first letters. Everyone is individual, and I would write different things to different people. I imagine we are already friends, or at the very lest, friendly acquaintances. I introduce myself through the conversation, even if it is through the mundane. E.g. the weather can affect many hobbies, and what you choose to do for the day: you wouldn't go swimming in the sea during a thunderstorm.
Others have said that they like humour and stories in letters, and the letters to have personality. A letter is a gift of time and energy. Letters should reveal enthusiasm, sincerity, and passion.
Where you write could be of interest. Part of the draft for this blog post was written on a train. That then begs questions - where have I been, and where am I going?
The original InCoWriMo was started by a fountain pen geek, and many participants were fountain pen users. Now, there are more participants who might not be into fountain pens, the colour of the ink (draft partially written with a Parker Reflex, Diamine Rustic Brown) and specific types of paper. Although I had fountain pens for use in 2008, I was using gel pens to write many of my letters.
What to write on doesn't really matter: a nice postcard; lined refill paper; or proper letter writing paper or notecards... Postcrossing expects you to send postcards of the same quality you'd be happy to receive yourself. There are some reasonable quality but not expensive letter writing paper out there. Paperchase may still have some writing sets in their sale!
