You named the big items which you need to keep up on maintenance. Plugs, O2 sensors, filters, change the fluids.
My GT has 196k miles and it still seems to do well on gas. A recent trip to Colorado, I got 27+mpg while in the mountains. On the drive there and back, it got the usual 24-25mpg at 80mph.
I changed my O2 sensors when the car had something like 150k miles. I remember it made a nice difference. Old O2 sensors can be slow to cycle, and thus don't do as good of a job keeping the desired AF.
Personally, I've never seen a reason to clean injectors unless there is actually a problem. The snake oils which supposedly clean your fuel system are just that.
A few comments:
Tire selection can make a big difference on gas mileage. Most performance tires have large tread blocks, and are designed for traction.. thus rolling resistance suffers. There are some performance tires that do ok on gas, but you more or less get one or the other.
Bald tires actaully have less rolling resistance than new tires. New tires will have deeper tread, and loose more energy due to treat squirm (deformation of the tread). Bald tires are also slightly shorter and will indicate more miles traveled than new tires, thus increasing your calculated mileage.
Synthetic fluids can help increase mileage, along with ligher weight oil. If your engine hasn't been getting synthetics, it probably isn't a good idea to change now. I still run full synthetics at my mileage.