Pretty much the better you keep the car, especially the often overlooked tires, the better it will run in general.
Shifting before secondaries open is going to help MPG, but I imagine that your secondaries would become gunked very quickly as compared to shifting often in higher rpms.
Exhaust definitely made a huge difference on my old mystique. I straight away went from averaging 480km/tank to 580km/tank. It did seem a little high, so I imagine there was something a little freaky, but you can't dispute a solid fact!
Increasing MPG isn't the only issue though, consider walking/cycling places too. Before I moved to Canada I used to walk everywhere. If the store where I wanted a packet of smokes from was 1-2 miles away, I would be happy to walk there and back (usually stop in at the pub though). Here I've found that my walking habits are changing as the general populace tend to like to drive to their mailboxes (some 100 yards away). Remember the average human's pace is something like 4mph, so a one mile walk will only take you 15minutes. Obviously it's not always practical, but every little helps.
