ypur flogging a dead horse with this one mate. Its a pretty uneconomical car in terms of MPG, the only way to get better out of it is to drive close to the optimum speeds for fuel economy. Thats basically lowest revs so highest gear, and stick to all the speed limits. Dropping from 70mph on a motorway journey down to 60 will improve your fuel figures, but you'd also die of boredom.
There is no 'magic' answer.
Probably the single most effective thing I could think of would be to fit some very lightweight wheels and tyres, and keep them 'skinny'.... as in not wide tread - 180 or 185 width. Keep them inflated to at least 35lbs. its going to be more productive than some dodgy additive or magic airfilter. Also, keep weight down in the car.