You might want to check your spark plugs. I've heard of several people having to replace them below 30k. I've noticed about .5 mpg increase since I've switched to 5W20.
The only other ways to improve MPG is to reduce weight and/or increase aerodynamics. Chrysler did a pretty good job on the 3.7L and it's almost impossible to squeeze anymore hp or mpg's out of it. Some other options would be a cold air intake. The idea is colder air is denser, the cheap ones don't work, you want one that has a heat shield, I recommend K&N. A betterflowing exhaust can help as well. At the moment no one make headers, so that would be your choke point and the intake and exhausts won't be as effective as they could be. You could also add an underdrive pulley, more efficient torque converter (if it's an automatic) or change your gearing. Throttle body spacers, chips and tornado kits (swirl the air as it's coming in) are snake oil, don't waste your money.
Most of the mods are done for omcreased performance, not increased mpg's. You will get a slight increase in mpg's, but if you are doing this to save money at the pump it's not worth it. You'd have to spend about $1500, for a very small (less 2 mpg, maybe less than 1 mpg) increase. Imagine how many miles you'd have to drive to offset the cost, well over 100,000.