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Chris,
If the advice above doesn't work (or even if it does you may want to check this), check for vacuum leakage at the base of the throttle body where it mounts to the intake manifold. Also check at the vacuum hose connections to the throttle body & manifold. Do this by spraying carb cleaner (like Gumout) there while idling. If the engine speed increases when you spray the Gumout, you have a leak there. You can also listen for leaks with a piece of heater hose as a stethoscope.
I had the exact same problem and found that my throttle body gasket was shot. The replacement part cost $4 and it took me less than an hour to replace it. The idle is rock steady now and I have gone from 42 mpg to 46!
In the Haynes manual the cause "vacuum leaks" was common to both the "high fuel consumption" and "unsteady idle" complaints in the troubleshooting section.
For a gasket scraper I used a piece of copper tubing flattened out, ground to an angle, and wire wheeled. This worked great on the aluminum surfaces. I applied a thin coat of Permatex Form-a-Gasket to both sides of the new gasket. For sure I have sealed the leaks, although this will probably make removal more difficult next time.
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Over...
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