Cars are like Humans, You Clog up our Throats we cant breath, and the Gasp for Air. In a Toyota there are a few things that can cause Idle Problems,
1) Fuel Pressure - this could be the fuel pump, fuel filter, or injectors. Since its a toyota, you can pretty much assume its probably not the injectors, but the pump is likely, but would also be noticed at wide open throttle.
2) IAC, Idle Air Contol Valve, What this does is allow air into the motor when you are idleing by signaling back to the computer what the Amount of Air needs to be. Of course this will not affect the overall MPG, unless you idle everywhere you go.
3) MAF, Mass Air Flow Sensor, this tells the computer how much air is in comming into the motor. These Go Bad alot, but will Affect your MPG Greatly if one goes bad.
4) the EGR system. Now, i dont want to get to far into this one because this only deals with Un Burnt Gas, and its really not needed, but will cause some Irregular Driving habits if it is malfunctioning.
My Recommendation
Take out the Idle Air Control Valve, Clean all ports with a Carb Cleaner.. Please Use Name Brand... I Dont want to see your injectors go bad because you used the Cheap stuff. Clean the Idle Air Control Valve Ends and make sure there are no carbon Deposits left. Use a Rag to make sure you get it all. And try not to Spray alot of Cleaner into the Intake Manafold, Spray onto a Cloth then Wipe... Spray only what you have to Clean out the ports, and let as much evaporate as you can before you try starting the Car Again.
One more, Very Simple thing that some Cars have Problems with is GROUND WIRES. If your Ground is dirty, loose, or you have too many things attatched to the battery your car will run funny.
Remember if you short out a spark plug nothing good happens.