What I found is that a lot of people report this secret fear that they're not really that smart, and it's only a matter of time before they're "found out". Like they'd tricked their first group into thinking they were capable, but the next one might be wiser and wonder why anyone ever thought they'd do well. They attribute the success they get to dumb luck instead of talent, and ignore evidence that they're really capable. In that respect it's different from low self-esteem, where you just feel bad about your potential. Impostor syndrome is about having evidence around you that you're smart, and choosing to ignore it.
It tends to afflict people when they're transitioning, in stressful situation, just basically out of their comfort zones. I also saw it talked about a lot in certain personality type forums - for instance, introverts are so used to "faking it" in social situations where they really feel drained by all the interaction, it's no surprise that they sometimes feel like fakes in other areas as well. Intuitive types, as opposed to the MBTI "sensors", are less likely to look at facts around them and more likely to want to analyze a big picture or read between the lines: let's just call them overthinkers anonymous and move on. They're great thinkers, but can get stuck in their heads. So if you took myers-briggs and your first two letters are IN, you might be set up.
It also plagues minorities who might naturally feel like they don't fit in. Society sends us all kinds of messages on what an engineer/scientist looks like - if we go against that, a part of our brains might be prone to get nervous. You can get the same effect in smaller circles, like families that label their children as "the smart one" and "the sensitive one"... as these kids turn into adults, they have to beat those labels to be who they truly are and that can be tough.
All of these secret fears can make one want to avoid challenges, procrastinate, even miss opportunities... that's what's so annoying about it. You can be so afraid of inadequacy you actually become inadequate.
The best thing to do about impostor syndrome is to surround yourself with good mentors who can accurately help you assess your strengths and weaknesses. In my presentation I actually used Hunger Games as an example... Haymitch might have been a raging alcoholic, but he woke Katniss up to the facts that 1) she could win but 2) she had to work on some charisma. Having a mentor to help you sort out your weaknesses helps keep you from inventing your own, or assume you're just one huge weakness poised to fail.
It also helps to put yourself in other people's shoes... realistically, they probably have better things to do than "discover your secret" of not being God's gift to your industry.
And who cares if you're not God's gift to your industry anyway? Most of us aren't in our fields to be THE BEST. There will always be someone smarter than you, always someone with more power than you. The only thing you can do is be yourself. Grow, learn, and love what you do... it's okay if someone else is better at your job.
Spend your time preparing for the best, not fearing the worst. Keep a file of your successes to flip through when you're down - the nice emails from coworkers, reassurance from your family. Maybe you can even pat yourself on the back for even having these fears of incompetence, it means you're outside your comfort zone.
And finally, help others. Mentoring is a great way to learn to give honest feedback, recognize what we have to get back, and give other people the words to own what's going on with them and maybe learn some more of those words yourself.