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.