Key to higher mpg is keeping the engine load high and the rpms low. That means:

1. When cruising, always shift up when you can. I found the Aveo can cruise in 5th gear at 30-35mph no problem.

2. When accelerating, use more throttle, but shift earlier. That doesn't mean to accelerate like a bat out of hell. It means to keep the rpms low enough that you feel the engine loading. When the car starts accelerating faster (lower engine load), shift to make the engine load higher.

3. Coast longer in gear instead of always braking at the last minute. When coasting in gear, the ECU shuts off the injectors and you're moving for free. That means keeping more distance between you and the car in front of you.

There's a lot more you can do for better gas mileage. But I feel this is a good starting point.