What modifications are good to help with gas mileage?

None. Get a good air filter, keep your air filter clean, keep your oil changed, keep your tires aired up, balanced, and rotated. Don't use the A/C or heater if you don't need to, drive reasonably, no start and stop driving, no speeding, no sudden stops, etc. Keep all maintenance up to date. No modifications are really going to improve MPG.