Driving style of high torque rise diesels is the one major thing that you can do to improve fuel economy.

Use progressive gear shifting - use only enough rpm to enable the engine to cope with the next gear up, and increase the rpm on each change up.

Know where maximum torque comes in on your engine rpm, know where maximum horsepower comes in, drive between this range as much as possible once the vehicle is at cruising speeds. All engines will also have a lowest fuel consumption rpm, usually close to maximum torque, but not always, drive with your rpm around the lowest fuel consupmtion rpm as much as possible.

Skip shift wherever possible - ie miss a gear or two if you are accelerating down a hill. Start in 2nd gear whenever possible - level ground or slightly downhill.

Use brakes instead of the gearbox to slow down - brakes are cheaper than clutches anyway. Skip downshift as well.

Avoid braking as much as possible - read the road as far ahead as you can see. Green traffic lights 400 metres away will only turn red, back off and coast up to the red light. Red lights turn green so increase speed to catch the green.

If you can fit an exhaust brake, fit it and use that instead of the service brake.

Replace thermatic fans with electric fans.

Aerodynamics - under front bumper air dams will improve fuel efficiency more than anything else you can add to a vehicle. The underside of vehicles are the least aerodynamic of the design.

Get rid of bullbars, roof racks, rear wheel carriers and any other non standard add on that roots up aerodynamics (as if).

If you must have roof racks use roof pods that are marginally better aerodynamically than a full length rack.

Fit a tropical roof and get rid of the AC - as if.

Get rid of PAS and build some muscles.

Dont buy automatics.

Keep tyres just slightly over inflated (1 or 2 psi) on highway driving. Rotate the tyres front to rear, side to side every 15,000kms.

Even keeping the vehicle polished will help a smidgen, same as getting rid of mud from the underside.