Optimum population is where the amount of resources available in a country is equal to the country's population needs, so there are enough resources to maintain its population.
If it is below its optimum population then it has more resources than needed for the population, if it is above then it has too little resources to maintain its population.
To achieve optimum population, a country must change some of the following dimensions to lower or increase their fertility rate, before they can achieve optimum population. Immigration, age distribution and changes in lifespan must also be taken into account.

Achieving Optimum Population
Social and Cultural
Changing people's views and attitudes on religion to adjust it into a modern fashion, changing social attitudes, such as giving women more rights and thought in starting a large family than following tradition.
Economic
Increasing career opportunities will have peoples' minds set on education and career prospects, and maintaining their job, such that the immediate impulse to start a family might be delayed.
Medical and Scientific
Increasing the amount of contraception in LEDC (Less Economically Developed Country) educating adults and children about sexual education, on how to use contraception and the risks involved.
Political
Improving education to direct people into a career, this will have people concentrate on getting a stable job rather than plan ahead on starting a family.