I'm going to guess that with 234,700 miles on it, things like cleaning the intake manifold will do nothing more than - - - make it clean. If you want to focus on the mechanicals, I suspect you'll do better by checking compression & depending on the results, maybe getting an engine rebuild.
Assuming the engine is fine and not needing a rebuild, having the latest and greatest coolant, battery acid, brake fuid, etc. probably won't get you anything, either. Having the lowest weight synthetic motor oil and transmission fluid CAN help, though.
Otherwise, make sure the tires are pumped up as high as you dare (I run mine up to 60, although anything above 50 is probably a waste).
Have you done any sort of grille block? I picked up a couple mpg by doing that, and since my front bumper cover/grille was black, just did it with black "gorilla tape" and it doesn't stand out too much.
After high pressure in the tires and a grill block, I'd say you can get the rest of it with your driving habits. Looks like your car is too old for a ScanGauge, which is too bad - those things are worth their weight in gold when it comes to the things you can learn about your car's "best efficiency spots."
Some general rules of thumb, though:
Eliminate as much high speed driving as you can. You mentioned thinking you did better at 55 than 60. I'll bet you do even better at 50!
Try to avoid idling like the plague. Doing things like rolling into the parking spot with the engine already off, and not starting it until you see the traffic is clear so you can immediately pull out onto the road will add up over the long run if you make a habit out of it. Also get in the habit of coasting (either engine off or just in neutral) as far as you can up to stoplights and stop signs. Make a game out of looking ahead to stop lights turning yellow up ahead & going into "coast mode" as soon as possible. It's amazing how far you can coast with the tires pumped up high - 1/2 a block is nothing. Sometimes I'll be taking off, going through the gears, from one light & then see the next one turn yellow. I'll just stop accelerating and start coasting before I've even gotten up to the speed limit on the street, and still make it to the next light before it turns green.
Finally, don't be intimidated by the impatient people behind you. If they want to switch lanes & "gun it" to pass you just so they can slam on the brakes for that same light you're coasting to, let 'em - - - even if they swerve back in front of you to rub it in your face they "won". They didn't win - they just burned a bunch of extra gas being an idiot. When you start getting those 50 mpg tanks, then YOU win!!!
