When it comes to reshuffling their Cabinets, prime ministers need to follow one maxim: if they think it ain't broke then it probably needs fixing.
Many a prime minister - especially one like John Key embarking on a third term in power - has paid a big price for failing to reshuffle their Cabinets and bring in new talent early enough or extensively enough.
Unveiling his new post-election Cabinet yesterday, Key was steadfast in declaring that he - unlike Helen Clark - was not going to be accused of failing to rejuvenate his ministry.
True, Clark was unusually timid by her standards when it came to reshuffles.
But then, there are few matters in politics more delicate.
Loyalty is a prized commodity. Dumping a colleague down the rankings is difficult. It is also dangerous in turning friend into potential foe. Then there is the reluctance to break up a successful team of senior ministers for an intangible called "renewal".
Then there is the question of experience versus inexperience. Do you take a punt and keep your fingers crossed that novice ministers do not outweigh the not-always-guaranteed benefits of renewal?
When it comes to succession plans, Key is much less constrained than other prime ministers - and for one reason. With three election victories under his belt and an increase in National's share of the vote each time, he is at the zenith of his power. In National's neck of the woods, he can just about do what he likes.
Moreover, the latest reshuffle leaves untouched those able to question his judgment - Bill English, Gerry Brownlee and Steven Joyce. Paula Bennett will soon be inducted into this elite club of advisers, if she has not been already.
But it is not solely a matter of power.
Key's reshuffle is more subtle than that. Somehow he has created the illusion that there are more winners than losers. He has done this by pushing a few ministers down the rankings, but only by two or three slots. At the same time, he has flagged to those ministers like Murray McCully, Tim Groser and possibly Nick Smith that this is their last stint in the Cabinet.
Key is also lucky - or makes his own luck. Judith Collins' sacking, Tony Ryall's retirement and Hekia Parata's demotion opened up three slots on National's nine-strong front bench to be occupied by what Key calls National's "new wave". Add Bennett, who was already on the front bench, to the names of Jonathan Coleman, Amy Adams and Simon Bridges - and four of National's nine-strong front-bench are "new wavers".
Just how much renewal matters when most people cannot list who is or is not on a party's front bench is a moot point. But voters can quickly tire of administrations in their third term. Key's strategy is to avoid giving them any reason for doing so.