[TimV]

Instrumentation.
Driving speed limits only highway drive slower.
Anticipate far befor your ride on trafic: drive like you have no brakes.
Partial grill block.
Mudflap delite.
Low air dam and/or tire spats. Front and rear.
Tires with low rolling resistance so you can use the weight of the truck in your advantage maybe smaller tires.
Small camback spoiler.
Bigger intercooler if its a turbodiesel.
Ecochip for more torque (Nm) ( probably not pocible cause its a mechanical pump?)
Oil pan heater.
Coolant heater.

I did everything what i wrote above, only need to install the coola t heater. And i raised my milage of my diesel from 8,5l/100km (29mpg) to 6l/100km (39mpg) thats a nice profit!