If it’s an emailed query and the clips are online, just add the web address where the editor can read the clip. If it is a mailed query, below your signature add a line that says that published clips enclosed. Then just add the copies of the clips to the envelope before mailing. Beyond clips, make sure you summarize your writing career in the query letter. It doesn’t have to be much – just to give the editor an idea of your experience and some of the more significant and recent pieces you’ve done.