There's a lot of great advice in this article, The Illustrator's Portfolio, including:
"When I look at portfolios, I don’t look for kids or cats. I look for your voice as an illustrator (“The Who”), and how well you execute your illustrations (“The How”). What should you put in your portfolio? Good art. That’s the answer. You should display art that’s executed at a high level. No matter what your visual handwriting is—from realism to abstract art—it must be done well. Show that you can convey a feeling or a narrative with a strong understanding of your medium."
Giuseppe also covers whether you should show one style or multiple styles, presentation (like how many pieces to include) and much more. Oh, and this smart tip:
"Straighten Out! Do yourself a favor: remove all of the factory-inserted pages and burn them. You’re already printing your work, right? So why not create a Photoshop template at the actual size of the plastic sleeve? Make the background black (or a neutral, or whatever you want). Then you can place your art digitally onto this template. Hit PRINT. Voilà! Every page will fit perfectly in the sleeve, and you’ll never again need to tape one piece of paper crookedly onto another piece of paper."He's even included a page template as a springboard for how you might organize your portfolio more like a book.
Go read the whole thing. And while you're at it, bookmark Giuseppe's Art Tips blog.
Illustrate and Write On!