As far as buying aftermarket parts for mpg purposes...Cold air intake, catback exhaust, lighter wheels...
As far as maintenance... Full synthetic oil (5-30 royal purple in my opinion) K&N oil filter, can go with K&N air filter that goes in the filter box, ngk iridium spark plugs and new spark plug wires, clean throttle body, and monitor the tire pressure try to keep it at whatever tire-recommended air pressure.
FYI I did all of things things, except exhaust and kept my stock 16's rims and I've gotten 19 city and 25 freeway provided that I keep my rpms under 2500k. In my opinion, a taco doesn't have to go above 80mph you get plenty of power and response under that.