As it turns out, I am seven days into working at a San Francisco online startup, and therefore I am an expert on these matters (yeah right). I have, however, become a bit of a job search expert by means of searching for a job for twelve months. While I kept pushing myself toward the stability of larger corporations, it always ended up that I received the most interest from startups. So, if you want to work at a startup and don't have any highly specialized skills that would make finding a job easier, here are some of the things that I think contributed to my getting the job.
Most HR personnel will see this as a red flag that you will burn out, or that you're not a very well-rounded individual. However, a true startup is going to be very excited by the prospect of someone who's really okay with showing up early, staying late, and probably taking lunch at their desk. They're going to like someone who loses track of time while immersed in a project, and who is stressed by not having enough to do. If you're sure that you have these qualities (or faults?), make sure you bring it up in the interview process. But don't sound like a crazy person, because they're trying to pick someone they'll enjoy spending those long hours with.
Usually you can fake interest in a company's product if you want the paycheck badly enough. Most people in the workplace are indifferent, at best, about the big picture surrounding what they do. This will not fly in a startup. You must possess true passion and a considerable amount of knowledge about the field, because you will be counted on to have a high base level of knowledge and be able to contribute specific experiences and opinions. Funds are carefully appropriated in a startup, and the passion that management holds for their product is so strong that they won't trust a paycheck to someone who doesn't share that passion.
Equal Parts Trigger-Happy and Cautious
My CEO says 'think fast, work fast.' You need to not be too scared to commit to an action without management signing off on every little thing. You also can't be stupid with that trigger finger. My parents inadvertantly taught me to live in a sort of "informed reckless" environment: if you stay aware, present, and informed of your environment then you can afford to be impulsive and a little bit crazy. They lived this way, taking huge chances but doing it in a way so that there were big returns (exactly the aim of startups). Big risk, big reward. But don't take stupid risks.
Jack of all Trades, Master of None
The way to stand out in a large corporation is to specialize to the point of being indispensable (the other option is to be an amazing manager, but you're more likely to be good at specializing your skills). The way to stand out in a startup as a young person is to be able to contribute from all angles, and at least get a handle on everything that is in motion. You will work with all departments and everyone must understand what the others are doing in order for the product to be successful. My arts, philosophy, nonprofit, public relations, and sales background was getting me nowhere at companies where the job titles are well defined... but in a startup I am valuable because I can grasp the overall process more quickly. You also have to look at it from the standpoint of who is doing the hiring: a startup can't know what its personnel needs will be two years (or less) from now, so someone who can be shaped into many different needs is going to be cheaper in the long run. If you crave structure in your position, rethink your desires to work in a startup, unless you have a Computer Engineering degree.
I had a ballet teacher who once said "Don't be a dancer if you can be anything else." This certainly applies to startups: they are a place for people who are not able to fit into the standard corporate mold, because they crave a bit more creativity and danger in their career. If you think you're one of those people, then by all means make sure you demonstrate that you have all of the afore-mentioned things while you are interviewing. And then, good luck. I hope those stock options get you somewhere!