I'm getting to be very comfortable on the monarch level, but I guess compared to a lot of people on this forum, I'm still a newbee... I build almost all of the improvements in my cities, but I build them in the order of their need and importance. I build barracks in all of my cities just in case i have to quickly produce a large quantity of units for defence or counter attack. However, I never suffer from a bad economy... I do find myself every once in a while short on workers, but I always manage to make 10-20 more in 1-2 turns and I am right back on track...
I would say that common mistakes would be building the cities too far away, not enough workers/unimproved tiles and lack of a clear strategy/goal/victory condition.