Gas mileage questions

I own a 1999 Oldsmobile Intrigue 3.8l engine with an estimated 130,000 miles.
I know the basics to get the most out of gas mileage:
1. Properly infalte tires
2. Wheel balance and alingment
3. Keep it tuned up
4. Drive the speed limit
5. Don't jack-rabbit to go
6. Drive with windows up
7. Remove anything from the trunk that will put more weight on the car
8. Advoid streets that have hills as much as possible

I've been reading about performace mufflers and they are supposed to help increase gas mileage. I asked a mechanic at a muffler shop, his response was, "Since the performance muffler puts less work on the engine, it will increase gas mileage."
So know, I want to get a second or more opinions about hi-flow mufflers.
Also, is there also anything else that I can do to help stretch gas mileage?