Maintaining a constant influx of people is not very difficult. The basic element of their satisfaction is work, so create new plots according to your plan of specialization or look at resources and try to fill up deficiencies. When new jobs are created, entrepreneurs will let you know which employees they need - then build several new housing plots.
However, remembering about everything that people need is impossible so after a while people demand the objects you've forgotten about - sometimes it is a shop, sometimes a swimming pool or a playground for entertainment. Executives and Elites provoke most problems as their level of compliance is much higher than the one of lower classes.
Remember about streets' passability - after reaching 50,000 of inhabitants public transportation becomes available. Buses can greatly decrease traffic problems if they are carried on in the right way. For the convenience of the residents, try to build as little as possible zones of unrestricted shape, choose rather rectangular ones as in rectangular plots bus stop reaching areas are much larger. The working class will be happy to benefit from such a means of transport.