I started my career at the startup and 2,5 years later I am still working with this company where I witnessed the expansion and growth of the company. At the beginning, the company had 1 manager and 3 developers. Now it has around 20 full time employees + some part time . Now I am development manager.
One thing we did wrong is - we did not control the expansion correctly. At the beginning - we didn't have hierarchy. We had developers and one of them was "primus inter pares" (who was formally company's manager). We had rapid expansion at one point, and since we were busy with doing the actual work we failed to see that the productivity level decreases with expansion that's not followed by the reorganization and that developers REALLY aren't good at selling and marketing. So after experimenting a bit - we hired two sales persons, divided the company in 4 departments and added 2 levels of hierarchy. Also the "sales" department is responsible for keeping eye on cost effectives of the other departments.
So now we have:
General manager
Heads of the
departments
Department staff
So my advise is to reserve some time at any given point in your company's life to reflect on the organization and to find ways to optimize costs and productivity. It's boring and tough job, but really pays off. And noone can tell you exactly how to do it - you must gain experience by yourself.