a google search found a nice checklist of things to do at:
http://www.misterfix-it.com/Solutions/5Air.html

"Service requirements for older furnaces
If your warm air furnace is old (say, 20 years or older), I suggest you
have a professional heating contractor servi ce it every year. This is
your best protection against carbon monoxide dangers and heating
problems. Routine service will also ensure peak efficiency to save you
operating costs.
A good service and inspection costs about $80 and should include a
complete cleaning, safety check, and tuning. The service contractor
should:


clean the burner and heat exchanger, and inspect for cracks.
remove the burners, clean them, and tune for proper combustion.
perform a carbon monoxide test in the heat exchanger.
check the vent pipe and draft diverter.
inspect the chimney for obstructions and draft.
check vent pipes for proper clearance and materials.
test fan controls and safety controls.
check the thermostat.
run the furnace through a complete cycle.
check flame roll-out.
check gas pressure (if appropriate for your furnace).
clean and adjust the pilot light assembly.
inspect gas fittings and repair any leaks.
lubricate the fan and motor.
check belt condition and tension.
service the filter.
clean the fan and housing if excessive dirt has accumulated.
Replacing parts or cle aning extensively will cost extra.
You should have a basic understanding of how the system works, so ask
the service contractor to explain the basics. You should routinely
service the filter and lubricate the fan and motor a second time during
the heating season. You should also inspect the flue connection to the
chimney. "

http://www.misterfix-it.com/Solutions/5Air.html