We are told that in 1918, at the end of the Great War to End all Wars, when the huge British conscript Army was disbanded, lost and bewildered men queued for mundane jobs, suddenly bereft of a sense of common purpose.

Richard Holmes’s magnificent book Tommy tells us: “It was a common sight in London to see ex-officers with barrel organs.”

Much the same happened, some years ago, to primary heads, when early retirement was an easier option than it is now. Active and assertive men and women, accustomed to commanding attention when they entered a room, discovered not only that they were ordinary mortals after all (“There is a queue here, love”), but that no matter how good the retirement deal, it still didn’t pay nearly as well as a salary.

None of them, to my knowledge, turned to barrel organs, though the late and much-missed Mike Lee, once a gleeful thorn in the flesh of Warwickshire authority, played the concertina in the streets of Nuneaton, albeit for charity.

Many became providers of in-service training. For a time, on any course you attended, speaker after speaker was introduced as “a former headteacher”.

It couldn’t last. Just how long can someone who’s cashed in their chips go on dictating the game? Warwickshire’s professional development leader at the time reckoned that the shelf-life of a former head was 18 months. It’s probably about 18 minutes now.

The problem has always been that primary headship can go on for a long time. A head appointed before the age of 40, has, in theory, a quarter of a century still to serve. For the first three to five years there will be juicy challenges - the coming of the new double glazing, the painless removal of Mr Gimcrack, the placing of a bomb under the caretaker. After that, what? Sooner or later there is the arrival at a plateau which, in reality, is an imperceptible downward slope.

In any other walk of life there would be somewhere else to go. But where? Teacher education is increasingly being done in school. Local authority professional development is done by serving heads. Another school? You’ve fought that battle before, and there aren’t too many who yearn to do it all over again, carrying five kilos of extra weight and your daughter’s wedding coming up.

At this point I should describe the exit route that I took, after 11 years of headship. In short, I put together one of those multi-task “portfolio” jobs. Mine included some writing, some lecturing, some supply teaching and a little private keyboard tuition. A year before I left my headship, a colleague head said to me: “A year from now you’ll be supremely happy and wondering why you ever hesitated.”

He was, of course, absolutely right. The balance of the portfolio has changed, but every day I still consciously stop for a moment to appreciate my good fortune. This attracts the interest of heads and teachers who wish they could do something similar.

The first thing to realise is that nobody owes you anything. Whether you want to write articles, teach on supply or deliver hilarious after-dinner speeches, what you offer must be in tune with what the buyer needs. A supply teacher, even a former head, cannot be a visiting celebrity, cosseted by the rest of the staff. A writer of articles must provide material that fits the publication - length, style, subject. Working in the public sector for years tends to make you forget these things.

You have to start preparing for your new life long before you leave the old one. In your last year, particularly, you should observe the old naval principle of “one hand for the ship and one hand for yourself”. If that means taking the odd day off to pursue contacts and opportunities, then so be it.

Take advice about money - whether to take your pension, what to do with a lump sum, whether to start a new pension. I made mistakes in that area, which I cannot now rectify and which I could have avoided had I talked in advance to professionals, including my union, about my plans.

Finally, be open-minded. I know former heads who now work happily delivering new cars. If you had to send a new car from Basingstoke to Glasgow, would you rather it was driven by a 20-year-old from the Job Centre or a 55-year-old respectable headteacher? (Yes, I know. Just keep the thought to yourself.) Gerald Haigh writes the Second Half column in The TES Leadership section, which returns in September.

Next week: Teaching abroad