“When motoring in a Mini, it’s important to occasionally take a left when you’re supposed to go right. In this way you can avoid ruts. If you feel like you’ve taken the same path so many times before that there should be a groove worn into the street, you’re in a rut. Take a left immediately.”

That refreshing bit of advice comes courtesy of Mini USA’s Book of Motoring, one of the many ways this automaker has marketed not just a car but a Mini mindset.

Mini ads are so quirky and fun, it’s as if Mini’s ad agency were channeling Ruth Gordon’s character in the “70s cult classic “Harold and Maude.” You can just hear Maude shouting out ideas in an ad meeting: “Break the rules! Take a chance! Live, live, live!”

On Sept. 1, Mini will unleash the highly anticipated Cooper convertible. Available in standard ($21,500) and S ($24,950) trim, the convertible will feature a power retractable soft top with an integrated power-sliding sunroof that can be opened and closed at speeds up to 75 mph.

Last year, Mini USA sold 36,000 Coopers and didn’t buy one TV commercial. Instead it spends its ad dollars on quirky billboards, magazine inserts, outdoor displays and the Internet. The real key to Mini’s success, though, lies in the Mini Cooper itself, which is as charming as the ads tout it to be.

The Mini lineup consists of the standard Cooper with a starting price of $16,999 plus $550 destination, and the Cooper S, with a $19,899 starting price. Both models are powered by a 1.6-liter, in-line four-cylinder engine. In the standard Cooper, the engine produces 115 horsepower and 110 pound-feet of torque and is fitted to a five-speed manual or a continuously variable automatic transmission.

In the Cooper S, the engine gets a supercharger and belts out 163 horsepower and 155 pound-feet of torque. The S engine comes mated to a Getrag six-speed manual gearbox.

From the outside, the S is distinguishable from the standard Cooper by a hood scoop, rear spoiler, dual chrome-plated tailpipes, S badges and 16-inch alloy wheels (the regular Cooper gets 15-inchers).

The Cooper cockpit is a busy place, brimming with circular and oval design cues. It is highlighted by such rally-inspired elements as a tachometer mounted on the steering column and toggle switch controls for the windows, locks and fog lamps.

The front seats offer ample legroom, even for 6-plus-footers, but the rear seats are more appropriate for stuffed animals and golf bags than humans. Trunk space is a minuscule 5.3 cubic feet, but it can be expanded by flipping the split rear seats forward. Curiously, the only way to unlatch the trunk lid is via the remote key fob; there is no interior or exterior trunk release.

For 2004, the Cooper and Cooper S receive only minor updates, including a new three-spoke steering wheel, a rear power outlet and a digital speedometer readout in the electronic display. A large swiveling cup holder has been added to augment the dinky fixed cup holders.

To celebrate the Cooper’s victory in the 1964 Rallye Monte Carlo, Mini recently introduced the Cooper S MC40. Mini will produce only 1,000 units of this model, which sports 17-inch anthracite wheels, commemorative badges, a carbon-fiber dash and the winning car’s number 37 on the door.

I spent a week scooting around in a Cooper S tester, and it was ridiculously fun. The steering and handling are go-kart sharp, the throttle response is quick, and the 16-inch tires stick like Velcro through the gnarliest of turns. Speaking of turns, the Mini’s small size and 17.5-foot turning radius make it the nimblest and most parkable thing this side of a motorcycle.

Both Coopers borrow some suspension bits from the BMW 3-Series – yes, this new version of the legendary Mini is built by German automaker BMW – but the S gets reinforced anti-roll bars and firmer springs. The stiff setup, combined with the run-flat tires, makes for a butt-bruising ride.

The Cooper might be small, but it has the kind of character and substance found in cars costing twice as much. It combines British charm with German engineering and an affordable price. Fun to drive, the Cooper is custom-made for people who don’t take life too seriously.

It inspires smiles on even the dreariest of days.