Government reshuffles rarely achieve very much. Even the most dramatic - the Night of the long Knives in 1962 when Harold Macmillan sacked seven cabinet members - did nothing to reverse the government's fortunes. Macmillan was out the next year, and Labour won the election the year after that.
Prime Ministers hate doing them. Of course you make some people happy. But you make many more unhappy - by sacking them, moving them, or, in the case of ambition backbenchers, disappointing them by not offering them a job at all.
And many believe they do little for good government. It takes ministers time to get to grips with their department. Why throw that knowledge and experience away? Would a business swap round all its senior jobs every couple of years?
But reshuffles happen. And this one will be trickier than most.
For a start, this is a coalition government, and the Liberal Democrats have five cabinet posts. They will almost certainly stay put. There is a clamour for David Laws to return to government (he is that are figure: a Lib Dem who is admired by most Tory MPs), but there is unlikely to be room for him at the cabinet table. Expect him to have a senior ministerial job though.
As for the Tories, the general consensus is that the big beasts will stay where they are - including Maidenhead MP Theresa May, the Home Secretary. More junior cabinet ministers - including Surrey South West MP Jeremy Hunt, the Culture Secretary - could be moved sideways. He survived the Leveson hearings, and oversaw the Olympics, and could be ready for a move. The Olympics Minister, Faversham and Mid Kent MP Hugh Robertson could be in line for promotion, although he would probably be reluctant to move - he has loved being the minister for sport. Sir George Young, the very popular MP for Hampshire North West could lose his job as Leader of the House if Mr Cameron decides to offer that job to Ken Clarke.
The Prime Minister will want to promote some female MPs. Basingstoke's Maria Miller is tipped by many for a Cabinet job, probably Secretary of State for Wales.
Lots of the new Tory intake, elected in 2010,will be clutching their mobile phones anxiously waiting for a call, but inevitably more will be disappointed than not.
Mr Cameron will want to appease the Tory Right without alienating his Lib Dem colleagues, a difficult balancing act.
Reshuffles also never go absolutely according to plan. There's always a hitch, and unexpected job refusal, something that means the jigsaw pieces never come together quite as the Prime Minister originally intended.
The other golden rule of reshuffles is that everyone who tries to forecast what will happen always, always gets it wrong.
You can find out more about the Prime Minister's Cabinet reshuffle on the ITV News National website.