1. Remember those gory details

However obscure your life may seem, you never know what the future holds, and what great things your past liaisons may achieve. They may even become prime minister.

Kept woman: Mae West

Ms Currie had no idea of the heights to which John Major's career would soar - as late as 1988, after their affair had ended, she still thought she could beat him for the leadership.

This naturally means that in years to come, there could be cash a-plenty in your otherwise mundane life. Snogged Claire from marketing? Got off with Jason from accounts? Get writing. The juicier the details the better - codenames optional - and don't worry about silly things like other people's privacy. As Mae West once astutely pointed out: "If you keep a diary, one day it may keep you."

2. A loyal friend

What starts out as lots of blank pages can quickly became your best friend. In times of emotional turmoil, committing your thoughts to paper is highly recommended by psychologists in helping to stave off anxiety and depression.

Enoch Powell said writing a diary was like "returning to your vomit"

It can also help you stay fit. Researchers at New York State University found the immune system benefits from writing about stressful situations, improving the body's ability to fight off infection.

In the closing days of her affair with John Major, Edwina Currie recognised just this factor, writing, on 28 September 1987: "Spoke to B this evening - I'm so glad he was in. Oddly enough, I need the diary more now that he's so busy."

3. Protect your side of events

Remember the spin doctor/Jo Moore e-mail /Stephen Byers row? It all seemed important a few months ago. Remember the row between Mr Byers and his press boss Martin Sixsmith? Byers said Sixsmith had resigned, Sixsmith claimed he didn't.

Alan Clark re-wrote the book for political diary writers

It mattered a lot to Sixsmith, not least because if he had been pushed rather than jumped, he would be entitled to a big pay-off.

But who to believe? Fortunately for Sixsmith, his elaborate diary of their conversations carried some weight. He got �200,000.

4. Enhance your own reputation

If you let it be known you keep a diary - or even better, avoid comparisons with Bridget Jones and refer to it instead as a journal - you may find suddenly people regard you with renewed respect. It will immediately mark you out as a thinker, a person with hidden depths who may in fact have a hinterland after all.

In this life, a guarantee of gravitas. And when you're in the next life, your record stands serene like the Taj Mahal.

5. Something for grandchildren

Can't keep your own diary? Write someone else's

There's never been such a thirst for knowledge about family history - witness the surge of interest in genealogy sites on the internet and how the 1901 Census online was besieged by millions of amateur family historians when it was launched earlier this year.

Imagine how much time and expense would be saved if we could all look back at our grandparents' diaries to uncover the secrets of their day. As the actress Dame Ellen Terry said in 1908: "What is a diary as a rule? A document useful to the person who keeps it, dull to the contemporary who reads it, invaluable to the student, centuries afterwards, who treasures it".

6. It's probably the only book you will ever write

One book too far

While many of us harbour ambitions to write a book "one day", it seems there are always better things to be getting on with. Writing a diary is low maintenance - just five minutes a day, no extra research required and after a few years, hey presto, it's the first draft of your autobiography.

However, it's not always a good idea. The work diary of Jeffrey Archer is one book he probably shouldn't have put his name to - it was used as evidence against him in his perjury trial.