There's a lot of tangents in this thread, but getting back to a list of sorts...
Nothing I mention here is new or extraordinary. Pick up a magazine article or book from the 1930's, 50's, 70's, etc., and it's there. I don't advocate risking safety for other potential gains but remember that driving itself is an assumed risk - there are dangers and if you know them you can do your best to reduce the risks. Some people are willing to assume more risks than others, so, as they say, "your mileage may vary".
1. MAINTENANCE! Basic, boring, routine maintenance is key. Know your vehicle. Know how it works, how it's supposed to run, what's normal and what's not. Knowledge is power.
A. Change the filters and fluids, use the proper oil, check the timing,
make sure your brakes aren't dragging, inspect, inspect, inspect.
B. Keep the tires properly inflated and your wheels aligned. Keep a decent set of treads and good suspension - it's safer and will allow you to confidently keep more speed through the turns (which in turn means less braking. See #4).
C. Listen to your car. Turn the radio off sometimes while driving and just listen - learn to recognize how your car normally sounds and behaves and you'll save yourself headache later.
2. Light is right. This came out of a book I once read somewhere, the author having quoted a race car driver of note. This has two valuable meanings for us - firstly (as they meant it) that your ride should feel "happy". Sometimes this means changing gears, sometimes changing speed (up or down) a MPH or two (or five or six), but never forcing it to do something that it just doesn't seem willing to do easily. Secondly, the literal transalation - reduce unneeded load. I'm not suggesting you toss out the spare tire (I consider that "needed") but if you golf only on Saturdays why drive around with your clubs in the trunk all week?
3. Slow down. You've heard it before. Many times. It's simple. Many of you find the idea objectionable, horrifying, even downright blasphemous, but if you
really want those big numbers, do it. (It's really quite easy).
4. NEVER use your brakes. Important exceptions: If you intend to stop fully, must adjust speed immediately and unexpectedly (someone cuts you off), or generally must cede to the laws of safety and sanity. Look ahead and be aware! On my daily 35 mile round-trip commute (excluding of course the above exceptions) I apply the brakes exactly TEN times, all of which are for full stops. Know your route and anticipate, using gearing and coasting to slow. Once you start driving this way you will despise those who drive in front of you, riding their brakes down hills and even up (!) them.
5. Get a TDIheater. This goes especially for those of us in colder climes, but your engine will thank you for it wherever you are, and you'll better your MPG to boot (assuming you remembered to plug it in).
6. Accesories. Don't use them. A/C? What's that? Oh, right, that annoying tangle of junk that takes up space in the engine compartment and weighs far too much...
6. Consider various mods. Most of these have been mentioned or covered already, but nozzles are a good place to start.
I have a '96 B4 Passat, very lightly modded, and if it doesn't return high 50's or low 60's something is wrong.
Fuel log here to view if you wish.