Start duct taping door seams once people get in the car, get your neighbour or friend to seal yours once you're in it. Don't forget the obvious like the bonnet, the gaps around the lights etc. Make sure your tyres are pumped up the bursting, in fact go for that ballon effect, less contact with the road, less rolling resistance. Remove the rear seats, or fold them down. That upright action from them isn't very aerodynamic. Don't brake, turn the engine off when going down hill. Disconnect all the fuses in the box to non essential things, ABS, Airbag etc, all of this eats power off the engine. If you can, just get someone to trailer your car about, this is a dead cert to cover miles without burning fuel.
On a serious note though, have you checked all the filters and that since you got it?