Editors will generally take a copy of the first page of any article you had published. This is also called a tear sheet by some editors. This tear sheet provides a sample of your writing plus shows information on the publication your article appeared in.
For convienence, I scanned tear sheets from three articles into my computer. This practice has allowed me to send to editors writing clips and the article by e-mail.
If you have not had anything published yet, you can still send a copy of the first page of unpublished works.