What I did...
I took five of my best essays--three of them were published in national publications and two of them in my high school paper--and I put the pdf files on Word. I changed the font and put in some of the photos from them, and then I added the title, byline, publication, and date, as well as a URL for online access.
It all really depends on what you're looking for. I felt that the small sample of my best work would be most presentable in that form. Plus, it didn't look like too much to read, with only seven pages worth of single spaced material. You want to make sure that whatever you send doesn't look unwieldy--they should want to pick it up and finish it.