In March 2010, Foursquare was riding high, one of the coolest social startups of the day, with gobs of fresh venture capital and a million people using its mobile app to check in. And then, on March 26, the company's website went dark. Somebody, it seems, had forgotten to pay $20 or so to renew the domain.

Foursquare's faux pas was quickly fixed; the bill was paid and the site went back up. But the blunder serves as a great example of how important it is, no matter how busy you are or how fast your company is growing, to write some things down. Indeed, it's during those heady days of startup frenzy that it's most essential to make sure important matters aren't left in somebody's inbox or, worse, some former employee's brain.

Write it down. Document your processes and protocols. Put them on paper. Like many of my favorite startup best practices, documentation has a long history among software developers, who are often asked to create documents that ride along with their software--the ReadMe doc being the quintessential example.

But documentation goes beyond software; it's good discipline, both for startups right out of the gate and for those soaring on a high-growth rocket. For the newbies, it's a way to inscribe your company's structures and strictures, to substantiate the firm you want to create. When it's only two people in a garage, documentation on core principles and strategies serves as testimony that this new thing is going to outgrow this garage someday. For those on the rocket, good documentation fuels your company with fluid communication and prevents it from being dragged down by cloistered knowledge.

There are four areas where documentation is invaluable:

1. Product development. It's amazing how rare it is that smart people scope out the who-when-why-how of a new product before they get started, given how much time and frustration two pages of planning can save.

2. Process. Each company has its own methods; committing these to paper (OK, not actual paper) as style guides and how-to protocols helps keep your best practices in circulation.

3. Onboarding. You spent months searching for your new superstar hire. And now you're gonna give her just a desk and a computer and expect her to figure it all out?

4. Customer support. Every business needs to have well-thought-out rules for how to communicate with customers. Docu­menting your answers as FAQs and internal support docs will keep your team consistent, efficient, and on message.

I learned the magic of documentation as a federal con­tractor when my startup, Iodine, was building OpenFDA for the Food and Drug Administration. We devised APIs con­necting outside developers to FDA data, so the data required explanation. But we opted to do more than the minimum, and went to great effort to translate the language of a federal bureaucracy for an outside audience. The more we explained the thinking behind the data and what uses it might be put to, the more people responded. Documentation FTW.

We've brought that bias to overexplain and overdocument into our day-to-day work as well­--and yes, there have been times when it seemed we spent too much time writing planning documents that nobody ever got around to reading. But that's a far lesser evil than the lost-in-the-funhouse feeling of the third meeting spent defining the same project.

There are some outstanding tools that make documen­tation fairly painless. We've used Google Docs, but, in my experience, which service you choose is less important than choosing one and sticking with it across the organization. Unless you really want to be the next Foursquare.