You can use a flat-blade screwdriver as described above but it does take some finesse. Best to use a mirror and a flashlight to look into the four holes on the back side of the steering wheel. The airbag has four steel posts which have slots cut in the ends of them. The steering wheel has spring-steel wire which snaps into the slots and holds the airbag in place.
You use the screwdriver to move the wire spring out of the slot on the post, while gently pulling the airbag away from the steering wheel on that corner - it will move out just a bit when it's released. Now, keep holding this corner out (yes, this is tricky, you can sometimes jam something into the gap between airbag and the wheel) while repeating the procedure at the other three posts. You'll know when all four posts are released as the airbag will pull straight away from the steering wheel.
Then you disconnect the airbag connector. You don't have to worry about the airbag going off when disconnecting it, as each airbag connector (including the one on the airbag itself) has a shorting bar in it which shorts out the two terminals when the connector halves are separated. This prevents static electricity or any other source of electricity from accidently activating the bag.
Posted just for those of you who don't want to buy the special tool.
