Yeah 18mpg is really good. We have a lot of hills here but I was able to squeeze out 17mpg (city) in my 4.0. Tune ups help as well as correct tire pressure helps. For me, driving like a granny and trying hard to keep the RPMs below 2k helped the most.
X2 nothing being around that is going to improve your MPG. Certainly that K&N isn't helping to do anything but allow more fine dust, silica, dirt, etc. into your engine. That's about the worst air filter there is for actually filtering dirt out of the air. They don't add any performance or mpg benefits to the TJ whatsoever, despite their marketing hype.
I got rid of my bronco that got 11 miles to the gallon for the jeep with 4.56 33s 3 inches of lift, and i get right at 18 and that is all city driving. havent figured highway yet. i cant complain really.
Aside from what everyone listed, which basically says you are already doing very well, you can get an all-season or highway tread tire in a lower weight and narrow tread width. Of course, the less aggressive the tire the worse it will perform off-road. So caveat emptor
Also drive as gingerly as possible. If you really want to know how easy you are driving, you can install a vacuum gauge and try to drive keeping as much vacuum as you can. Generally, the higher the vacuum the less fuel/air you are using, and the better the mileage.
Mindset, skillset, toolset - in that order!