The biggest changes will come from changing driving habits, but there are a few low-cost car modifications too.
For the car:
1) Keep your tires aired up. Going a little higher than is printed on the placard is a proven way to improve economy, but you will also have a harsher ride. I run 45 PSI in my car, but even 38-40 would be an improvement over most cars' factory 32-35.
2) When it's cold out, block off part of your grille to help the engine warm up. A piece of cardboard or plastic works great for this. Be careful not to block your intercooler.
3) Get a fuel economy meter. The instant feedback will save you more money than anything else you can do. Ultragauge or Scangauge are good choices.
As for driving habits:
Think about maintaining momentum, and don't burn gas racing to a redlight. If a light turns red in front of you, take your pedal off of the accelerator. You'll save both fuel and brake pad life. Everyone will leave the light at the same time, regardless of when they arrive. In stop and go traffic, try to drive at the average speed of traffic, so you're not speeding up and slowing down. When your car is stopped for a while, turn it off, idling burns fuel. Also, stay off the turbo, the engine goes rich and it burns a lot more fuel.