1. Don't send unsolicited pitches to Marvel or DC Comics.
The panelist explained that, for legal reasons, Marvel and DC have a “no unsolicited pitches” policy. In order to avoid being accused of stealing the ideas that are sent their way by hopeful young creators, they simply don't look at anything that they didn't specifically ask to see. Creators will be better served soliciting their ideas elsewhere, which leads us to...
2. Do pitch to Image Comics, Dark Horse, and other smaller publishers.
While the big two have to be insular to avoid lawsuits, publishers that focus on creator-owned works often have open submission policies. Dark Horse doesn't accept submissions based on their licensed properties (likely for the same reasons that Marvel and DC don't accept unsolicited submissions at all), but they do take original pitches based on original characters, and Image Comics only publishes creator-owned works. Conveniently, they both have their submission guidelines published online.
3. Be personable.
Trying your luck with a cold pitch sent to a publisher remotely probably isn't going to hurt your efforts, but its no substitute for putting in some face time with the people your hoping to work for. The panelists recommended getting out to conventions, going to after parties, and networking to meet some of the editors and professionals who work in the comic book industry already. You can't send in cold pitches to Marvel or DC, but who knows, maybe if you talk to an editor or another creator for a while, and they get to see what you're all about, they'll actually ask to hear your ideas. That said...
4. Don't go for the bathroom pitch.
As Danny Fingeroth pointed out, just because you're in the bathroom at the same time as Mike Carlin doesn't make it a good time to start pitching. Editors are people too. They need space, and following them into the next stall over to tell them all about your ideas for Superman probably isn't going to make a good impression. Be personable, but don't stalk an editor and then try to force a pitch on them when they least expect it.
5. Cultivate an online following.
While you're waiting to get your foot in the door with one of the established publishers, publish some of your work online. Its a golden age for web based and digital comics and these kinds of projects can help you create a fan following before you ever get your first job with a publisher. Use Twitter, Facebook, Tumblr and other social media to build a rapport with you fans. Building a fan following shows publishers that people like your work and likely want to see more. This goes a long way towards proving to them that you have what it takes to be an industry pro.