1. Your art should look professional. This is actually a multi-step process.
---Clean up your image before scanning or before having it photographed. Erase any smudges and trim any unsightly borders. After scanning or photographing, if it still needs to be cleaned up, do it in Photoshop or another photo editing program. There's no need to see spills, smudges, fingerprints, or dust marks. Make sure you scan your work properly at 300dpi for print quality and save a copy under a different name at 72dpi for web purposes. Be sure to adjust for color and contrast (and that your monitor is calibrated). If you have digital details to add, like the covers of the books, do it now. You can see the difference between the unaltered scan and the image that has been adjusted to match the drawing.
---Choose a professional presentation portfolio. A few years ago, I purchased an 8.5x11 Pina Zangaro black presentation book. It screws open and closed to allow for additional pages, and has clear, archival sleeves with black backing, just in case the picture does not fill up the entire 8.5x11" space. It's hardcover, durable, protects my art, and looks like I care.
--Know your audience. This particular portfolio selection was made for a Children's Book conference, but it's important to know your audience. Trying to get graphic design work? Don't show pet portraits. Trying to get illustration work? Leave out professional design work, like logos, and stay away from images that don't tell a story, like simple portraits.
---Find the best order for your works. This is not my final layout, but I was still trying to determine order and which pieces to include. Arranging it out on the floor helped me to visualize sequence.
---Don't neglect your best dummy images. My illustration group gave an excellent suggestion: do not neglect to add the color spreads from your dummy into the portfolio itself, just in case the editors and agents do not have time to look at the dummy.
--Start with and end with pieces that pop. Why not make your best first and last impressions?
--Show both color and black and white. Here I really speak from experience. Last time, I had an entire black and white portfolio, and the feedback I got? People were unsure if I could handle color, if I was unwilling, or even afraid to do it. Since they could not see how I used color, they could not visualize it, and just so. Each person uses color so differently that there is no real way to visualize how one artist will execute in color. So show what you can do--in color and black & white.
In preparation for this idea, I made dividers for the color section and the black and white section, joined by theme and execution.