That's easy to say, harder to do.
1. Don't over rev, change up gears at about 2500rpm;
2. Always keep looking ahead so that you can anticipate changes in road conditions and alter your speed accordingly eg when on the open road, you see in the distance an obstruction, slow down now, to give time for the obstruction to move. Slow down in anticipation of a reducing speed limit. Take your foot of the gas sooner when coming to a roundadbout.
Reading the road and anticipation is an important part of good mpg;
3. Don't go over 60mph - the aerodynamics of 4x4's are awful and cause increasing drag as the speed rises. But it's not linear ie 10% increase in speed, 10% increase in drag, it's exponential (I think that's the right name??!!), 10% speed increase - 10% drag increase, 20% speed increase - 40% drag increase!!
So, cruising speed has an enormous effect on mpg.
4. Don't expect car-like performance. These vehicles weigh roughly 2000kgs, with engines that provide about 55-60 bhp (for diesels) at the road wheels (loads of power loss through the transmission) whereas an average Modeo 2 litr Tdci gives about 100bhp at the wheels and weighs about (depending on model) 1400kgs. They WILL drive more dynamically !! Forcing a Disco along will kill mpg, especially on an auto.
Basically, relax when driving your Disco, start your journey 5-10 mins earlier to get there on time and enjoy the comfort.