How to structure your illustration portfolio? What to put in? I've just found this very useful post written by Cheryl Klein, a children's book editor from Brooklyn. She explains in detail what she likes to find in an artist portfolio, whatever your style might be. And even if you're not aspiring to become a children's book illustrator, I think it still worth a reading tho.
These are passages I've found interesting the most:
-Illustrations involving animals, either anthropomorphized or real -- whatever your style is best suited for. I would suggest that you have at least two or three of the following common picture-book animals somewhere in your portfolio: a dog, a cat, a dinosaur, a cow, a pig, a chicken, a duck, a horse, a rabbit, a wolf, an elephant, a mouse, a tiger.
-A few (3-4) pieces with the same subject, ideally a few consecutive spreads from the same story (extra points for having the text on the page). This could be an original story or a familiar text -- Mother Goose rhymes and fairy tales are good choices for their familiarity (though I must say that most first-time illustrators will probably not be able to get either a Mother Goose book or a fairy tale retelling published in an overcrowded market). This allows me to see how you handle the same characters in different perspectives, positions, and situations; what parts of the written narrative you choose to highlight in your picture; how you transition from one scene/emotional atmosphere to another; and how you choose to advance the story through your illustrations.
-If you have an interest in doing book jackets for novels, make some mockups of new jackets for already published books. I'll throw out three covers that could be fascinating to revisit: Eragon by Christopher Paolini, Twilight by Stephenie Meyer, and Saffy's Angel by Hilary McKay. (Not that there's anything wrong with those original covers -- just that they provide a wide range of subject matter and characters for you to reillustrate.)
Go here to read the whole post.