It's harder than it sounds, and it sounds impossible.
The path to an in house counsel position at a startup company isn't too different than any other type of company: BigLaw is basically the path you must follow. If you're fresh out of law school, and unemployed, your best bet is to either (a) spam BigLaw firms with your resume, (b) stand outside of their offices with a "please hire me" sign, or (c) find a job as a paralegal or contracts administrator and hope that you end up in the right place at the right time.
But, if you've done your time in the BigLaw trenches, and are considering fleeing to greener pastures, here are a few resources that can help:
"Why Startups Hire Their Own Lawyers"
In this excellent article by Daniel Doktori, a WilmerHale associate, he explains why startup companies make the decision to go with in house, rather than outside counsel.
The short version? The bills expand beyond the cost of hiring one salaried attorney.
If you have any interest in making the in house jump, the article is worth reading, as it explains the unique role of general counsel in a start-up environment (as opposed to a stodgy corporation), provides tips for success, and most importantly, helps you understand what the companies are looking for. It's like a scouting report for Silicon Valley and other start-up hotbeds.
"Real Advice About Hiring a Lawyer for Your Startup"
Go for the most qualified (that you can afford reasonably), go for the best subjective fit for your company, rather than the most credentialed "best" on paper, and finally, youth should not be a factor. ("Luddites and technophiles persist at every age level.")
It's great advice for entrepreneurs, which will help you, the applicant, to shape your application.
Start Up Job Listings
If you can't find a position through networking, there are a number of startup-specific sites that have listings for in house counsel, such as StartUpHire and VentureLoop. A brief perusal of the listings confirms our earlier statement: this is territory for experienced BigLaw-yers only.
Learn the Business
Though you'll hopefully learn much of what you need to know about seed stage financing and convertible debt at your previous stops, the absolute most comprehensive resource out there for startup law is Wilson Sonsini partner Yoichiro "Yokum" Taku's Startup Company Lawyer blog, which exhaustively covers financing, stock options, M&A, incorporation, and issues between founders.
Editor's Note, February 28, 2017: This article was first published in January 2014. It has since been updated.