I do a lot of flying -- mostly as a airline passenger, but also on Flight-Sim (cheaper that way)
I read an article that American Airlines went on a quest to reduce fuel consumption a few years ago. The plan: reduce as much weight as possible.
This translated to removing non-essential equipment such as food warmers, calculate the proper amount of fuel + reserve (no more), and going as far as to justifying a profit from the in-flight magazine, or it was to get the boot.
Lightening the fleet saved the company loads of expenses. IIRC, about 10% fuel savings. As Laz mentioned, single-engine taxi is very common among all airlines. I've been in cases where the on-board power unit (a turbine-powered auxiliary power unit, or "APU") isn't used to cool the aircraft in the summer, and passengers are asked to pull the shades to help keep the cabin "hot" instead of "blistering". The big, yellow ground-air-con hose can't often handle the demand. It gets toasty, but fuel is saved...
Here's the biggest problem: With the current Air Traffic Control setup, a flight isn't "direct" -- meaning a flight from Chicago to LA isn't a straight line. So controllers can track flights with their antiquated technology, airways have to be followed, which are like "expressways in the sky". The final route will look like a jagged edge instead of a straight line. This process leads to congestion, overworked controllers, delays, vectors off-course for spacing the aircraft for landing, and speed restrictions. All of this adds up to more fuel consumed. A priority needs to be placed on updating this system, or it's only going to get worse.
Every transportation industry is feeling the fuel price pinch, and that is passed down to the consumer. Baggage weight limits have decreased, with surcharges for over-weight bags increasing. This is all with the average American weighing more every year.
Same goes for the engine itself. Northwest is the best example. I often fly on the old Douglas DC-9. The engines are old, loud, and inefficient -- but I'm sure the 35-40 year-old aircraft are paid off (they use the "Low-Bypass Turbofan" engines -- very cylindrical in appearance). In contrast, the newer Airbus A319 (similar to the Boeing 737) has a quieter, high-efficiency, "high-ratio bypass turbofan" engine. They're have the more common large intake. I'm sure they still carry a note on those...
To survive, I'm sure the trend will continue...
RH77