One clever trick is to leave a subject open. Put in some information that almost begs a question. It leaves the reader itching for more detail. When you finish your letter, go back over it. There is always a chance that you skipped a word, phrase or whole sentence in the eagerness to get the text down.
Nothing is more frustrating than losing the whole crux of a story through one crucial omission.
Interesting e-mail should strike a balance between the different subject matter. Unless you share a common hobby with your E-Pal, you could bore that person with one long diatribe, particularly if the subject is of interest to only you. Turn your subject matter into questions. Make it less obvious by not making the question too direct. If you want, you can use the anonymity that e-mail provides.