The only viable way to preserve a constitution against the tyranny of an aspiring prince is to have governence by constitutional assemblies of the nobility and armed “commons.” Augustus convinced the Senate to give him (and his successors) lifetime power to head the civil, military, and tribunal offices in addition to being head of the religious authority as supreme Pontiff. This essentially made him the law in addition to being the law. Even with so much military and civil authority being vested in one person, for 200 years from August to Commodus there was virtually no outbreaks if internal bloodshed htrough military might.
In order that the Empire wasn’t left headless upon the death of an emperor, he was able to “adopt” another man as his son in the line of Caesar and vest with them some, or all, of the powers of the empire. During Hadrian’s time, he chose a fifty year old senator named Antoninus Pius and a seventeen year old boy named Marcus (later Marcus Aurelius) to succeed him. This pair was known as the “Antonines” and ruled for 42 years with wisdom and virtue. The reign of Nerva through the reign of the Antonines could be considered the happiest the world has known