Poor gas mileage. How can I improve it?

Recently, my Ford Focus has been getting some pretty crappy gas mileage. I thought that maybe it was because I needed to have my oil changed. I DEFINITELY had to have that taken care of, especially since the guys at Valvoline Instant Oil Change goofed on the mileage when I last went there to have the oil changed before the most recent visit. They were off by 20,000 miles before then. Since I got the oil change, I've felt a restoration in performance, but noticed no restoration in gas mileage. Before and after I got the oil change, I've been thinking of other things that could be harsh on the gas mileage *besides* occasionally using a little more gas mileage than I ought to use. Even when I don't do it, the mileage is *still* kinda low. My dad suggested that the air filter might need to be cleaned out. So I took it out and looked at it. It didn't look too bad; it wasn't really filthy or anything-- just some debris in the crevices, so I shook the filter out to get rid of any loose debris, cleaned it out the best I could, put the filter back in, latched the air filter housing back down, plugged the sensor back in, then went driving later on. Cleaning out the filter didn't do much good. My gas mileage *still* sucked. Well, I recently replaced the OE air filter with a K%26amp;N washable, reusable high-flow OE replacement filter. No improvement in gas mileage... yet. I probably still need to break it in. What else can I do to restore mileage?Poor gas mileage. How can I improve it?
I would change the spark plugs and perhaps the wires too if they look old. Weak spark will not let the car burn all the gas, as a result, you use more. Try a fuel injector cleaner also if it fuel injected. I am sure it is, but check it anyhow. If that does not get you a bit more mPG then take it to a shop. And check your air pressure in the tires!Poor gas mileage. How can I improve it?
tire pressure, plugs and wires, fuel filter, injector cleaning(take it to the shop not that autozone crap). air filter,replace o2 sensors all of them(can get pricey). most of all driving habits ther is nothing more you can really do. oh and air is 78% nitrogen that is a belle tire scamPoor gas mileage. How can I improve it?
Fix the air pressure in your tires.

Learn to accelerate slowly.
first you need to define bad mileage with before and after numbers. air in the tires is negligable at best and the k%26amp;n filter is doing you more harm than good. An oil change, unless you were on the verge of being out of oi,l wont change performance. Is there an engine light on? Sometimes there are codes present that will stop the computer from running rediness monitors and that can cause a faulty part to not be flaged and no light will come on. Also to consider most cold markets use a diffrent formula of fuel in the summer and winter your computer may be taking a little longer to respond to the change.