By Laura Smith-Spark BBC News

Queen Elizabeth II has won praise for her dedication to duty

Ruler of a long-established constitutional monarchy and head of a Commonwealth embracing countries as diverse as Australia, Kenya and Canada, she is widely respected around the world.

And while there is a low groundswell of support for republican movements in the UK, the prevailing mood is not one in which the Queen need fear for her future.

Meanwhile, on the other side of the world, Nepal's King Gyanendra has grabbed the headlines with an entirely different style of rule.

After two weeks of violent mass protests over his seizure of direct power from the country's elected government 15 months ago, he has responded with an offer to return power to the people.

But it has yet to be seen whether Nepal will return peacefully to a constitutional monarchy.

Concede power

So what is the secret to a successful reign?

HAVE YOUR SAY The late King Hussain was one of the most outstanding world leaders. I miss his wisdom and leadership. Ahmad Hmoud Send your comments

Historians suggest that the key lesson for royals seems to be handing over power to the people sooner rather than later.

This is a lesson the British monarchy learned earlier than most, says Professor Denis Judd of London Metropolitan University, with the public execution of King Charles I in 1649.

When, after an 11-year gap, the monarchy was re-established, Britain's royals over time had to accept a largely symbolic role - and in so doing, managed to survive through centuries during which many European nations became republics.

Britain's King Charles I was an early victim of people power

"People don't want even enlightened autocrats. They want firm government but they want a government that they can throw out after a certain number of years," said Prof Judd.

"With a monarch you have to wait until they die, unless you have a violent revolution.

"The British monarchs, like the British aristocracy, have been very good at conceding enough power to keep their financial privileges and status."

'Disillusioned people'

Surya Subedi, professor of international law at Leeds University, agrees that the desire to cling on to power can lead to downfall.

King Gyanendra's assumption of direct rule, prompting protests in which several people died, was "unwise" and has done enormous damage to Nepal's monarchy, Prof Subedi said.

Nepal's King Gyanendra reversed his country's constitutional monarchy

His action reversed the decision of his brother Birendra - murdered with his family in 2001 by the then crown prince - to become a constitutional monarch following a people's uprising in 1990.

King Gyanendra sought to justify his takeover by saying it was necessary to quell insurgency by Maoist rebels, and also pledged to put his people first.

"He promised a long list of things that he hasn't been able to deliver, so people are increasingly disillusioned," said Prof Subedi.

"The respect the people had for the monarchy is more or less gone.

"They see how other countries are developing, see they are doing economically well, and see the monarchy as not serving the people - it's an expensive white elephant."

Many wives

Prof Judd also draws a link between the economic strength of a country and the fortunes of its ruler.

King Mswati III of Swaziland has been criticised for his lavish lifestyle

"If an absolute monarch can deliver the goods, then they have a good chance of surviving," he said.

He cites the example of oil-rich Gulf rulers who have kept their subjects relatively happy by spending money on infrastructure and public works.

By contrast, Africa's sole remaining absolute monarch, King Mswati III of Swaziland, has faced criticism for leading a lavish lifestyle with 13 wives while many of his subjects live in poverty.

Prof Judd says Europe's constitutional monarchs have been perhaps the most successful in adapting to modern expectations by moving away from their elitist heritage.

Many European royals work in relatively ordinary jobs, he says, and a recent trend of princes marrying commoners is evidence of a pragmatic view of the future.

God or man?

Sometimes changes to a monarchy have been driven by external rather than domestic forces.

The 1947 Japanese constitution, drawn up following World War II under guidance from the US military occupation force, confines Japan's emperors to a purely ceremonial role.

It came a year after Emperor Hirohito stepped forward to announce publicly that he was human and not a divine being.

Norway's Crown Prince Haakon broke tradition by marrying a single mother

"Some people took it quite hard," said Dr Hiroko Takeda, of Sheffield University's School of East Asian Studies.

"People in the war had died believing that the Emperor was god and the fact he was not turned everything upside-down."

The wedding in 1959 of the then Crown Prince, the present Emperor Akihito, to a non-royal, and the ensuing modernisation of the imperial household helped people come to terms with the changes, Dr Takeda added.

But perhaps rulers should look to Thailand for a example of enduring popular constitutional monarchy.

The world's longest-serving ruler, King Bhumibol Adulyadej will mark 60 years on the throne in June - and is widely revered by the Thai people.

Meanwhile, the crowds drawn to Windsor for Queen Elizabeth II's birthday walkabout have given their verdict on why Britain's monarch is here to stay.