Under the Roman Republic and the Early Empire, the Praetorian Gaurd served fairly honorably in defense of their Senators, and later their Emperors. Later on, however, they became increasingly involved in the Empire, killing multiple competent emperors and instating many horribly incompetent leaders, simply because they wanted more pay and favors. Similarly, the Legions came to expect a pay increase pretty much every time an Emperor died. This led to the emptying of the Imperial coffers and a huge rise in instability lasting into the late Empire.
How can this political instability be avoided? How can the Praetorian Gaurd and the Legions be made subordinate to the Emperor, instead of the Emperor subservient to the demands of the former? How would this affect the position of the Empire later in its life?