Are there any simple mods or something I can do to increase my gas mileage? :?

three letters: CAI

You can't just buy one of those can you lol? I've read the thread that says how to build it, but I don't know if I understand all of it to actually do it. I'd like to though

A simple K&N replacement filter will increase your mileage as well.

A tune up.
New plugs, wires, O2 sensor, and a k&n air filter.
Make sure your not throwing any codes too. For exapmle, a knock sensor code will pull timing, or an egr code will tell you your egr valve isn't functioning properly which will drop mileage.

Do you know the dimensions of the factory filter?

pull it out yourself, its pretty simple

For O2 Sensonrs, which is the best brand?

AC Delco ONLY. And certainly not Bosch. That'* the worst for us.

Best thing to do to increase fuel consumption beside driving less is to go easy on the loud pedal. Also turning off the engine if idling for more than 30 seconds. Accelerate as if there was an egg between your foot and the gas pedal. Coast a lot and in the highest gear possible. Keep the boost gauge as far below 0 as possible pulling the highest vacuum.

.......and if think I follow any of these rules then you're dreaming.

That'* probably why I only average 15 mpg.