Step 1: Get a topper or cover for your truck
Step 2: Reduce weight
Step 3: Re-route air intakes
Step 4: Driving habits
When you are approaching a stop light, let the car coast for as long as you can. If you are far away and it is red, sometimes you can time it in a way that you won't even need your brakes. This will greatly increase mileage, considering that it takes the most energy to accelerate form a dead stop.