See where you do most of your miles.
In the city, you'll benefit most from weight reduction and driving technique.
Check out the tips here :
100+ Hypermiling / ecodriving tips & tactics for better mpg - EcoModder.com
On the open road, drag (and speed) reduction helps.
Not using a roof rack is a good choice - these are really a drag.
Speed - on longer trips the reduced fuel consumption will not pay for lost wages due to being in the car rather than on the job.
Increased tyre pressure helps everywhere - crank it up to the psi for max. load for starters, then increase bit by bit.
The ride will get harder, but should never become bouncy.
An air dam is a none-too-obvious feature to cut drag
These vans are already fairly well streamlined as vans go.
Can you get these with a manual transmission ?