Each time you position a residential zone make sure it is covered by the required city services. Otherwise, the building will eventually be abandoned and will not generate any taxes. When placing fire stations, police stations, and other services, take note of their range. Try to get the most coverage by not having any of it extending into areas that cannot use their services, such as corners. Factories and service buildings do not need to be covered by firefighter or police protection. Keep smelly buildings such as factories or sewage plants as far away from residential areas as possible. As long as they are attached to a road of any length, they will still service your city.