Without a 'bluebook' of guidelines on car prices and values in Hong Kong, as there is in most major car markets, figures for ultra-luxury models are hard to come by.

But a browse through the web and newspaper ads found a second-hand 2007 Bentley Flying Spur going for HK$1.79 million, a 2007 Porsche 977 priced at HK$1.1 million and a 2003 Ferrari Maranello being sold for HK$890,000.

A rung down on the ladder, however, among the executive-class saloons, the price of a new BMW 7 series ranges from HK$1.07 million to HK$2.67 million, while a 2002 model is being sold for just HK$140,000, 'or nearest offer'. BMW's 5 series come at a starting price of just over HK$556,000, while an owner is trying to sell a 1998 model for a snip at HK$25,000.

This is bad news for owners of cars over three years old who are looking to trade them in for a new model. It also illustrates the importance of taking appropriate measures to ensure your car retains as much value as possible.

The most important thing is to make sure it's regularly serviced. If it's still under warranty, make sure you have it serviced by the original dealer.

Keep it clean: that's another golden rule. Assuming a car has been regularly maintained, a potential buyer is looking for a motor that simply looks good.

'Keeping the car's value is probably about what you can see from the outside, what you see when you sit in the driver's seat - a seat that's not torn, for example,' says Gary Parker, a committee member of the local MG Car Club. 'The value is in the eye of the beholder.'

Windebank says: 'Have the car properly valeted, or detailed, as they say here, at least every six months. What kills a car here is the humidity. It gets into the fabric and the leather and the carpet, and it does start destroying it, whereas if it's properly cleaned and valeted, it does prolong the car's life.'

Few local car owners are able to spend a Sunday afternoon cleaning their own cars, especially if they live in a high-rise, so finding a good car wash is critical.

'You want somebody who's going to wash it by hand. A lot of places claim to hand-wash but they use a machine,' Windebank says.

For non-Cantonese speakers, asking for a polish may be lost in translation.

Ask for your car to be polished and you may get what's called a T-cut in other places. This means using a cutting compound to remove the old wax, after which the car is rewaxed. Your car will gleam, but it's more expensive.

Paintwork may become more vulnerable as pollution concerns increasingly push carmakers to use water-based paint, which scratches more easily.

'You need to find a really good heavy-duty polish or preferably one of the new coating systems [that have come on the market],' Windebank says. One product that's recently become available gives the paintwork a polymer coating, which is 'self-healing'. If a car gets a minor scratch, the coating slowly merges back into a smooth, shiny surface.

Respraying is not generally recommended, but if you get a small stone chip while driving, you should cover the area with a paint pen to prevent rust setting in, Parker says.

He also says that changing the engine oil at least yearly is essential to keeping a car in good shape. The high humidity makes this crucial in Hong Kong, especially if you leave a car standing unused for a long time.

'When your engine's not running, air will get into it, and after time that air will release water into the engine,' he says. 'You could start getting water into the oil, and the iron parts will start to get rusty.'

Keeping a car off the road for extended periods can cause unexpected problems, Parker says. Rust can eat away at ball bearings or the crankshaft.

Customising is an area to stay clear of, as the additional expense pumped into the car to modify it to your own specifications may not appeal to other buyers, who will likely prefer the vanilla model.

'If you're looking to buy a car every one or two years, it probably makes financial sense not to go for the bolt-on goodies,' Windebank says. 'It might only be a matter of personal taste.'

Parker says conversions to avoid include fitting bigger wheels. Changing the wheel radius can lead to problems with braking.

Younger motorists should also avoid the temptation to fit a new exhaust pipe to make the car sound sportier. If the engineer has removed the catalytic converter while changing the exhaust, the car might not pass its next service.

Apart from the obvious harm that can befall you and your passengers, a serious crash will also devalue a car, however well it is fixed up. The higher the speed, the more the chassis could be damaged.