Fuel economy in your MINI is affected by several factors, all of which are under your control. Here are some tips for improving your MINI's gas mileage:
Observe the speed limit - the faster you drive, the more gas your MINI burns. Driving the speed limit is a good way to conserve fuel.
Reduce Air Conditioner usage - the AC system puts a load on the engine which uses more fuel. Open windows when driving at less than highway speeds. (see below) Use AC when at highway speeds.
Keep Windows closed - Open windows create aerodynamic drag at freeway speeds which uses more gas than using AC with the windows closed. Use AC at highway speeds. Open windows when driving at less than highway speeds.
Take less congested route - stop & go driving is very inefficient. Take a less congested route even if it's a slightly longer route.
Avoid excessive idling - turn engine off if you sit still for more than a minute.
Use up the whole tank - don't fill the tank until it's below 1/4 tank. If you like to keep your tank above the 1/2 mark, your MINI is hauling more weight in your tank!
Confirm proper alignment - If your MINI is not properly aligned, you increase rolling resistance. Make sure your alignment is up to spec.
Choose harder compound tires - Modern 'high mileage' tires tend to use harder compound rubber (with less aggressive tread patterns). Performance tires with softer compound tires tend to have higher rolling resistance.
Use the recommended grade/viscocity of Motor Oil - yes, using the wrong viscosity can affect your gas mileage! Look for the Energy Conserving API label. This symbol indicates that the oil is certified, fuel-efficient oil by the American Petroleum Institute (API). MINI approved oils are in SAE classes 5W-40 and 5W-30.
MINI diet - if you car is loaded with unnecessary 'stuff', unload it! It takes more gas to move more weight. (You can also improve gas mileage by switching to light weight alloy wheels with standard tires. The stock MINI wheels and the runflat tires are quite heavy.)
MINI recommends the use of unleaded premium fuel with a minimum octane rating of 91 (R+M)/2.
Above all, drive sensibly and safely. Plan your driving so you don't make unnecessary trips. Carpool when possible.