Usually I don't write back immediately, so I have to reread the letter and see what that person sent to me. My reply usually depends on what I received: if someone sends a beautiful postcard, I try to find a nice one too. And if someone sends me a whole package, I'll send a package back!
Here you can see a beginning of a letter.
My letter is almost finnished, I've found some things to include (I'd like to send vintage ephemera such as vintage stamps and old papers, tea bags (in this picture: tropical fruits, my favorite!), air mail themed things and.. well anything suitable that I find in my boxes).
I love wrapping and beautiful packages so I put all the extra goodies in a cute polkadot bag.
When my letter is completed I put it on a scale (I bought this little letter scale in a thrift store) to see which amount of stamps is needed.
I used old stamps (from 1980-2000, they are still allowed - you just have to put enough stamps on your letter to get the right amount of postage), it doesn't only look better, it's also cheaper! I get mine at postzegelsmetkorting.nl.
And when my letter is finally completely finnished, I put it on a pile of letters waiting to get posted (I usually post a couple of letters at one time).
How do you write your letters?