I took a company from 2 people to 80 when we were at the height.
There have been a lot of attempts to crack that nut and most have been unsuccessful.
First you need to realize what is going on when you grow.
1) The more people you hire the more you erode the culture you had. Therefore be very mindful of what kind of people you hire and be quick to realize if they don't fit the culture (even though they might be good). I.e. hire slow, fire fast.
2) Have a set of principles that make you comfortable. If you don't like bullshit have a no bullshit rule, if you don't like agression have a be polite rule. The point is that you will need to be able to see yourself in those principles.
3) Always make it your failure that people are leaving and lear what you can do better next time. Even if it's a person who were really bad for the culture it's your failure. Even if they leave for a better job it's your failure that there is a better job. Find ways within your capabilities to deal with these failures.
4) Most importantly. Make sure people know they are there because they are good enough. I cannot tell you how important this and how many great cultures it has destroyed. Doubting leads to sub-optimale work. Instead of creating a competitive environment create an environment where people feel safe and cherished, that way they will perform much better, at least in my experience.
Last but no least. I rarely make book recommendations but Ed Catmulls (from Pixar) book "Creativity Inc" deals with this exact problem. I can't recommend this book enough. It's so filled with wisdom and perspective and as a bonus a surprisingly good mini Steve Jobs biography.
To give you some of the insights from the book I can recommend you listen to this talk he did http://ecorner.stanford.edu/authorMaterialInfo.html?mid=3299
That might give you some things to look for and see if you can find ways to address them whether you run the company or you work for it.