As a general rule, putting a lift kit and bigger tires on a truck will decrease its gas mileage. There are, however, some ways to mitigate this.
Tires and suspension components have an indirect effect on the gas mileage of a vehicle. 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. Mud terrain tires are for hardcore off-road enthusiasts. The more aggressive tread 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.
As far as the lift goes, a suspension lift alone can affect drive shaft length, steering geometry, brake lines, gear ratios and overall weight. Obviously, the lighter the lift that will get a truck to its desired height, the better.