Ive played with a few things over the years, just in case I ever needed something extra (like for a really long trip or higher gas prices).
The first thing I tried with a lot of success was over pressurization of the tires. I either usually push it to sidewall max pressure or a little over. Ive seen as much as 2.5mpg gains from this and Ive never had a tire issue. Granted, you dont want to run at the track like that, but for fuel economy, it works well.
Next, pay attention to the traffic lights. Coast where you can. No sense in accelerating down a hill when you can coast and save gas. I do not, repeat do not recommend putting the transmission in neutral. In a manual transmission, maybe, not in an automatic. Come to traffic lights like your brakes failed. You dont have to be the first one to the light and the first one off the line.
If you are running a conventional motor oil, try a high quality true synthetic motor oil. I picked up nearly 3.5 mpg in the GP from this alone.
The ignition system has always been good for one or two miles per gallon for me using low resistance wires and more powerful coils.
As you mentioned basic maintenance goes a long way, but thats been so drove in the ground that most are probably familiar with it. A good tune is another place some mpg's can probably be had, especially if you are running rich.
Prior to the GP tranny failing I had hit 34 mpg on the highway prior to the tranny taking its toll on mileage. It started life with me getting just barely getting 28 mpg all highway and was more often in the 27 mpg range.
My Isuzu Rodeo started out barely getting 17mpg and I drove it like that for a month, my wallet a crying, to make sure that would remain constant before I started any work on it. Last week I hit 23.6 mpg. You do the math. When I first got it, I would fill up on Monday, then have to fill up again on Thursday. Now I can fill up on Monday and still have fuel left over to drive around on weekend in town for a bit depending on what I need to do. And I have yet to switch it over to synthetic fluids. Just this past weekend did I switch the transmission over to synthetic fluid. I still need to do the engine and rear differential and I have not even started on the ignition system yet.
My goal is to be consistently in the mid to upper 20's with the Rodeo.