Culture at a startup is like capital -- once you've started running out, it becomes harder to raise more; and once you're out, you're done.
I've twice worked at startups that doubled in size within a year. The first time, it was bad -- for morale, for productivity, for overall quality. Yammer has been really good on all of those fronts. Why? Retaining your culture as you grow is not due to luck or accident, and it's not (necessarily) correlated with your company's success trajectory.
I can't take credit for any of this -- it existed before I came -- but the critical elements seem to boil down to:
Identify the critical elements of your culture. There can only be a few "most important" things. Be explicit and obvious when talking about what they are.
Preserving the critical elements of your culture will not happen automatically. Adding new people, changing processes, forming sub-teams, interviewing and recruiting and promoting -- all of those can threaten your culture.
Look for changes constantly and figure out why they're happening and if they're beneficial. Figure out what behavioral or organizational things you're doing that might be leading to undesirable changes and adjust.
Executives have to care about this stuff and talk about how culture is critical to the success of the company. There will be times when expediency or "because I said so" is tempting, but that is a massive hit to culture. Your company may not weather more than one of these interventions.
...That said, it has to be everyone's job to absorb culture, care about it, and actively preserve it. That means questioning things that aren't working. It means when you have a coworker who is being a jerk, instead of everyone trying to ignore it, someone takes them aside privately and explains why it's not okay. It means turning down candidates who have great skills but no particular appreciation for the company's core values.
At Yammer, you'll see people who were hired two weeks ago interviewing new prospective employees and asking the same questions or having the same concerns as far more senior employees.
(That's one of the main reasons I joined: everyone -- and I interviewed with a really varied slate of people -- had really similar -- but honest -- assessments of what made the company successful and what was needed.)
Now that Yammer has been acquired by Microsoft, public opinion has mostly been that all the good things about us will be stifled and go away. But as a manager, it's my job to fight that perception -- and reality -- as hard as I can. That means doing all of the things above on a much larger scale -- educating, questioning, pushing back... and hopefully extending our culture barrier out to include a couple more thousand people here, a couple more thousand people there.
3+ years post-acquisition: has it worked? Well... yes and no. I think we've done a reasonably good job of most of these things - preserving transparency, constantly retrospective-ing our people and dev processes, communicating and over-communicating. Yet it'd be disingenuous to claim that pre-acquisition culture has been maintained. The sense of urgency has faded, and with it, sometimes, a belief in our own abilities to effect change.