AirFilter

Air filter maintenance

Remove the seat ( I sure hope you know how to do this)
Remove the gas tank. (Or loosen it. see section below). This may require loosening the side fairings on pre-2003 GS500s to reveal the bolts holding the tank in place. You actually don't need to disconnect the hoses if you can get a screwdriver in to remove the air filter. See below for more details
Examine the air filter. In all probability, if you don't live in a dusty climate, it will look normal. If it is coated with dust, expect to spend slightly more time.
Using some form of compressed air (computer duster gas will work), blow the dust out of the filter from the OUTSIDE to the INSIDE (on the stock filter: others may be different!!): see this thread. That is, if you see where the air enters the filter on its way to the engine, blow it out from the opposite side. If you blow compressed air through the filter it will embed the particles more firmly into the filter and weaken the airflow to your engine.
- GSTwins member seshadri_srinath has used gasoline to clean his air filter before with good results. Simply soak the filter in gasoline and let air dry.
If you have a K&N filter, apply the special filter oil to the outside as needed. See http://www.knfilters.com/cleaning.htm
Replace the filter, tank, and seat.

If you find that your filter is clogged, or simply want to upgrade your bike, see the Air Filter upgrades page page.

More details on the gas tank fiddling, from the above referenced thread (presumably for stock sized filters) Kerry writes:

You don't need to mess with any hoses to get your air filter out:

 (edit: you may or may not want to run the gas tank level down before fiddling with this :)  )

Use your ignition key to remove the seat.
(1989-2000 models only).Undo the 10mm bolts holding the side plastics on
(1989-2000 models only).Pull the side plastics out of the grommets on the frame and let the front drop down a little.
Remove the 12mm bolts that attach the rear of the fuel tank to the frame. (On the OUTSIDE side of the frame, not the little things under the tank)
Pull the tank an inch or so toward the rear of the bike so it will clear the frame in front.
Lift the back of the fuel tank and support it on a short 2x4 laid crosswise on the frame.
Remove the 4 air filter mounting screws and pull the filter out.