Udi Dahan has written a nice post about Scalability.
I agree with him that Infinite Scalability is a myth.
Scalability costs and actives users count is different and dynamic as compared to Total Users Count.
My thoughts are
1. Know You System
Know the bottlenecks and have solutions ready. If you system is not scaling well with increase of active users or requests/per user, you should be able to give solutions to management. I have seen how people’s career has suffered because they could not provide answers to management.
2. Measure, Measure and Measure
Know you system's vital statistics
3. Follow Good practices
Good Practices never hurt. If x number of industry experts are saying that Y is a good practice. Follow Y practice if one don’t time to evaluate it. Hardware Scalability generally costs money where System Refactoring /Re-writing requires more cost and money.