Depends on what tires you have now.
There is very little a driver can do with regard to the suspension of his truck to increase gas mileage, but there is a lot he can do to decrease it. Being smart about the lift kit and tire combination chosen can save gas mileage without too much performance loss. There are several factors to consider.
The most important decision to make is the tires. All-terrain tires are all-purpose tires designed for both street driving and strong handling in moderate off-road conditions. For casual off-roaders, all-terrain tires offer better gas mileage and longer tread life.
Wider, heavier mud terrain tires have a more aggressive tread which increases rolling resistance to put more tire on the ground, which is great for off-road but can significantly reduce gas mileage. For weekend off-roaders, a set of middle ground all-terrains should be sufficient without hurting on-road gas mileage too much.
If you are running a highway tread tire, that's going to be optimum as far as helping to preserve your current fuel mileage.
One other affect of lifting a truck is introducing more air flow under the truck, which is not as aerodynamic as the top so some losses will be experienced.