At comic book conventions, a lot of young artists hope to "break into" the industry by showing editors their work. It sounds simple enough, after-all, you just need to draw a few pictures and they'll tell you if you're good enough, right? Well, no.
The most important thing to remember is that just as not all comic books are the same, not all portfolio reviews will be either. Editors from different companies will have different motivations for holding a portfolio review. Some might give "soft" reviews to artists with the ultimate aim being public relations. Others are on the lookout for fresh talent, and can get rude if they don't find what they're looking for after sitting down with a number of lackluster artists. So what should you expect and what should you use in your comic art portfolio?
There are, of course, a number of different answers, but the most important one is to draw pages of sequential storytelling. We'll get into specifics later, but this is one that's so universal and obvious that I have trouble believing that too many people show up with much else besides sequential pages. The most important thing is that they want to judge your ability to tell a story. If your perspective, anatomy, etc. aren't top-notch, they'll come with enough study and practice, but the most important thing, is your ability to convey a sequence, without dialogue, and have the reader (in this case, the editor) tell what's going on.
The next question everyone has is how many pages should go in a comic art portfolio. There's no clear-cut answer to this, (unless they specify on the website that they want at-least five consecutive pages or something) but a good rule of thumb is anywhere between three and fifteen. After-all, if you're just kinda showing it to people at booths without there being a portfolio line or a formal call (and especially if you're just showing it to artists looking for advice), they probably aren't going to want to sift through a dozen pages.
And yet, if you're showing your work to a well-known publisher, three or four pages might seem a bit thin. If you're traveling, though, I'd recommend about twelve and you can whittle it down from there if you get enough negative feedback on something (as long as it's not in the middle of a story).
Next, there's the issue of subject matter. I'd say the best thing to do is draw what you're passionate about, but be weary that there might not be as much demand for what you do. If you have standard superhero stuff, Marvel and DC may be so swamped with artists dropping xeroxes of their work off at their booths that you're lost in the crowd, while other publishers want to avoid working with artists who might think that all there is to comics is Batman and Spider-Man.
Conversely, if you know a certain company is there that publishes only horror stuff and your main goal and skill revolve around drawing Archie, it may be wise to forget them, rather than slogging through six pages that you can't stand looking at. That won't get you a job, and if it does, it won't be one you enjoy.
In a lot of articles, artists are usually advised not to include any pin-ups (read: single illustrations) in their portfolio while others insist that it's helpful to see if they can handle a mock cover. The best thing to do is include a couple of pin-ups (especially if they're things that have been published) and, if you're including multiple stories, use them as a divider. They may not tell the proverbial editor much about your storytelling, but hopefully the rest of your stuff will.
Another thing to remember is that most of the larger companies want artists to display work that is drawn in pencil, as if another artist would come in and ink it later. This seems to be less important to them now than it was years ago, but it's still something of a standard practice. Smaller, independent companies, on the other hand, might prefer if you do ink your own work, so if you have work from school that you put in your portfolio, it might be worth taking with you.
Beyond that, a lot of the obvious rules of any networking or job interview apply. Don't look too slovenly, have some business cards printed up, and do some research about the companies who will be looking at portfolios. After-all, there's nothing worse than a talented artist not getting a gig because he was rude or unprepared.