Upvote:2
Centralize power
Part of the reason the Chinese (Han-Ming) were able to achieve vast expanses was the tradition of strong bureaucracy and limited power of nobility. Exams and merit-based officeholder selection kept this viable in the long-term. By contrast, Western European courts were entrenched in centuries-long rivalry with the aristocracy who boasted fortified positions in their castles and maintained regional autonomy. This meant that the royal courts had their fiscal incomes diluted. Similarly, with the Japanese Empire, a strong sense of national identity accelerated its concentration of power at the central level and arguably facilitating its territorial reach. Flattening the nobility took decades not centuries like in Europe.
Soft power
This makes for a convenient segway to another subtle yet key element to governing a large territory for long periods of time: cultural cohesion. Again using China as an example, although it was annexed repeatedly by northern Steppe powers, it was the refined soft power of the Chinese Han that persisted in the following centuries. Groups that annexed China, Manchurians and Mongolians, became to a large extent "Sinofied" during their reign. An extensive literary corpus, civil engineering and state planning capacities may ensure that even if your fictional empire is temporarily overthrown, its "DNA" will not be lost. Many still contend that China has the oldest continuous civilization despite these interruptions in the nation's sovereignty.
Learn from past mistakes
I would caveat that while China has all the above virtues, it was still prone to major issues in policy implementation. Due to vast distances of the empire, there are many accounts of changes that were ordered by the central government only to be discovered, years later, that those orders were never carried out. China never really resolved this issue, and depending on the technological level for your setting, this may need careful calibrating to avoid. Other issues include inefficiencies stemming from the fractal design of the bureaucracy. There were multiple contending bodies across central and local nodes in the hierarchy over the same projects. An example being irrigation/water: a general central government water project body, a local government water project body and a central government water project body designed specifically to align the local body to the central governments interests. It can become very nebulous very quickly.
Upvote:4
Well, the Romans wondered the same thing, and starting with Diocletian began splitting it into two empires with two (theoretically) co-equal Emperors, with the Western half including western North Africa, Italy, and Western Europe, and the Eastern half including the Balkans, Egypt, and Asia.
This map shows the boundaries in 395 AD.
The Chinese only really reached that geographic expanse that posed this kind of problem during the Middle Ages. The Tang Dynasty mostly dealt with it by setting up a powerful civil bureaucratic system, and ceding most of their day-to-day power to it. So they weren't exactly "totalitarian", in the sense that most political decisions were being made by local career officials, not by the one true font of power off in the capital.
The British during their Imperial era mostly used a system of local Governors to act as the monarch's local representative and make any required quick decisions on their behalf. However, during most of this period the kingdom was being run under some form of Constitutional Monarchy, with elected bodies like Parliament really having most of the power, not the Monarch.
It also helped greatly that communications during this era was much faster than in the Ancient and Middle Ages, and the British put a premium on improving it. This shows up in things like their obsession with Naval technology and their downright possessiveness of the Suez Canal (which they did not build and originally had no ownership of).