Well, that would good to answer, too.
But Cheka's question has definitely some merit: if a ruler can live much longer than the average man, assuming he can't just fireball to ash any opposition, what are the measures one could employ to keep the throne permanently? Think of all the vampires, liches and assorted baddies of fantasy - they wouldn't want to loose power, if they gained it, right?
Let me put down some thoughts on the topic:
- solid knowledge of the principles so eloquently described by Mr. Machiavelli cannot hurt, whether it is a matter of instinct, or sagely knowledge
- 'progress' seems to me like a fairly modern concept, but there is some relevance to the idea - new thoughts and innovations are necessary, but also have a tendency to threaten the status quo, hence the need to be in control of them. Historically, kings and the like were supporters of innovation that advanced their goals (as entertaining toys, but that is beside the topic). So one needs to position himself as sponsor of curious minds, while restricting them anywhere else.
- at the same time, while needing to keep people from wealth unless it is by king's help, there must be a productive population that can support any goals of his.
- choosing peace as the initial relation with all other countries may be an interesting option - anyone attacking the country will be subsenquently destroyed. A non-aggressive ruler that 'keeps to his people' may have a more solid position that one going for world government - but hey, why not try it, if you have the time?
- if it is safe from the point of keeping immortality, sire children, and plant them all into important positions, marry to important families, etc. Over the centuries, your family should be everywhere, their position and wellbeing depending on you, so they should act accordingly. Be careful with the idea of successorship, though.
- support a religion/philosophy/teaching that makes the king the greatest guy under the sun (connecting him to gods is also fine); also teachings that make people complacent and accepting their position in life.
- speaking of it, this should be put more generally: a ruler that rules already for a LONG time, has to be silently present everywhere, so that the country is defined by his existence, and is hard to imagine without.
And so on.