Pick Your Presentation Pieces.
The presentation of the portfolio itself aside, let's focus on the actual content. Your presentation portfolio isn't intended to be a 200-page visual novel of your artistic prowess; instead, it's a sample of the best work that you want to display to potential employers or others who might have reason to view your work. I'd recommend keeping the number of pieces in your portfolio between 18 and 24; 20 is the ideal number.
You'll want to pick your pieces based on two things: quality, and the skills that you want to highlight. If you're only interested in showing off your painting skills, then pick your 20 best paintings and scan them (or high-quality photos of them, if they're too large for any available scanners) for your portfolio prints; if, however, you want to show off your paintings, your 3D texture map composites, your Adobe Illustrator digital art, your motion studies, and your CGI lighting projects, then pick your four best of each and include those in your portfolio as demonstration of the wide range of your talents.
Remember, however, that not everything is marketable. If you absolutely can't stand to leave a piece out but you know that potential employers will frown on it, save it for your bound sketchbook.
Arrange Your Pieces to Complement Each Other.
There should always be some kind of order to the arrangement of your portfolio pieces, even if it's just simply ordering them by type. But you'd be better off organizing them by quality, so that they cushion each other, the stronger pieces providing support for the weaker ones. Space them much as you'd space clips in your demo reel, with one of your better pieces first and your best piece last, with more of your better displays interspersed with some of the "just good enough" ones to keep from creating long stretches of impressive artwork followed by long stretches of less-than-interesting pieces that might leave a lasting dull impression.
Include a Bound Sketchbook.
The sketchbook is the catch-all for all of the artwork that got left out of your portfolio, and a way to display the diversity of your talents more fully. Almost anything can go in your sketchbook; mine is a massive collage of nearly 300 pages of pencil sketches, landscapes, musculature studies, cartoon breakdowns, figure drawings, motion studies, object studies, lighting experiments, CG art, a few photo manipulations, and even various renders of 3D projects from toying with lighting, morph targets, and biped animation to create idle poses that pleased me. Sketchbook work doesn't even have to be completed work; it's more a gathering of your ideas, and sometimes employers prefer looking at sketchbooks over portfolios, as it gives them a glimpse into the raw developmental process of your work.
The sketchbook, like the portfolio, should never be composed of originals; instead the originals should be scanned and printed onto standard letter-size paper (you can get away with printing these on your home color printer) before being bound into a standard notebook. You can usually get them ring-bound or spiral-bound with vinyl covers for under $5 at your local printers, depending on the thickness. You can tote it with your portfolio anywhere; just slip it in the inside pocket and then if, during an interview, someone expresses a desire to see more, you can offer the sketchbook.
Your portfolio is just as valuable as your demo reel; you should put just as much time and effort into its composition as into your multimedia presentation. The two together are a formidable pair, and should be used to present yourself in the best way possible.
