Nissan Repair: gas mileage, poor mileage, nissan altima

Question
I have a 1994 Nissan Altima and as of lately I've noticed that I'm getting very poor mileage. I know the car is old and in time it will give me poor mileage but I feel that this happened too sudden as if maybe something is wrong that should be checked out. If I drive for 10 minutes a quarter of the tank is gone. It was never that bad. What could it be? I was told one thing to check is the air filter. Can you give me more things to have checked out or changed.  

Answer
Perform a tuneup on the car.  Change the plugs, wires, cap and rotor.  Make sure the tires have the correct air pressure in them as this will really screw up the mileage.  Also, think about changing to a K&N filter when you change the air filter.  The K&N is more expensive but you can clean it and it lasts for years.  The K&N filter will get you about one mile a gallon more than a paper element.  I forgot to mention that you should change the gas filter too.  And, if you have not done so change the PCV valve.