At the time I proposed using small flash games as a means to showcase some of these critical elements (as well as the ability to prototype, a critical skill for game designers, imo) along with some other key support material. I wasn't particularily satisfied with my answer, though, because clearly not every game designer is going to be able to do the programming and artwork necessary for pulling together a flash game single-handedly.
A week or so ago I read an excellent article by Brenda Brathwaite on exactly this subject entitled The Game Design Portfolio: Is There Such a Thing?. The article is concise and well written and answers the question much better then I did during my talks.
I really recommend the entire article, but here are a few choice quotes:
"Make games. Any kind of games. Flash games, board games, card games, collectible card games. Use whatever you have at your disposal and make a game out of it."
"...the person with the prototype is going to win every time."
"...don't start in January for your May job search. Start now for the job you want four, five, or 10 years from now. Seriously. Set up a blog and start writing about games intelligently and regularly."
"...if it's not good, don't show it."
I could go on, but really I'd just be quoting the entire article. If you are reading this blog and attended any of my talks in Vancouver, please do yourself and your career a favor and read Brenda's article. It could make all the difference in your attempts to get your foot in the door.