Let us just look at the beginning of the system of khilafah.
Prophet Muhammad pbuh spent all his life to preach islam. He made himself living example of excellent character so that those who could object otherwise, could see the practice of all the rules and laws of islam. When beloved prophet pbuh was bestowed prophethood, he pbuh ws living in a society which was rich in culture and progressive economically. The tribe of beloved prophet Muhammad pbuh was the chief tribe so mainly, in the political issues his tribe was given importance and weight. In such an established society, starting something new was virtually impossible, but Muhammad pbuh made it possible and world saw it. What he (pbuh) actually did to achieve this virtually impossible goal? He (pbuh) had no force to back him but his family name and few loyal and loving relatives. Surely, this was far from enough to change the very foundations of an established society. He (pbuh) worked with a strategy which was workable. The taks was tough but His (pbuh) wisdom made it a supper win for him.
1: Allah swt bestowed prophethood on beloved prophet Muhammad pbuh was he was forty years old. Before his (pbuh) prophethood, he (pbuh) still was a well known man, for his (pbuh) character and qualities. Therefore, the first thing in establishing khilafa is the need of strong character of the people who want to establish khilafah. In a characterless community, no one will ever find the need of change or would appreciate if a change is proposed.
2: Prophet Muhammad pbuh remained humble and patient when he preached. He (Pbuh) was beaten and mistreated but He (pbuh) never talked back or retaliated in the same manner. Therefore, second thing is patience. Nothing can be achieved with arrogance but it is patience with which even wars can be won.
3: Prophet Muhammad pbuh preached with strong will and never thought of quitting no matter how many difficulties he faced. He (pbuh) never took a step back, rather He (pbuh) stood fast and proved to be a real leader at all times. Therefore, strong will and real leadership qualities are required for khilafah not just debates and talk shows.
4: Prophet Muhammad pbuh never quit preaching even when He (pbuh) got huge success. Which leads us to yet another important finding that achievement of goal is not success, rather sticking to the goal till the last is the essence of a real success.
5: After the departure of Muhammad pbuh, His (pbuh) companions did the same, they got all their lessons from quran and sunnah. Instead of fighting, they refered to what ALLAH and His prophet Muhammad pbuh has given them and so they succeeded. This takes us to the finding that, establishment of a system is not a one man's job, rather it should be taken to next level and should be kept alive by collective efforts. One man simply dose his job, the followers keep it alive.
Therefore, loud slogans, long talks, books and seminars, they all cannot make even a difference of a shred if the society we are living in, is not prepared for establishment of khilafah. Continuous striving for this task is the requirement. Selflessness and honesty is the need.