I'm also a former STL-area resident. My favorite place when the 12s are active is Bridgeton Station Rd. There's a wide paved shoulder on the west side of the road where parking is allowed and you can even get out and climb up the noise abatement mounds between Lindbergh and Bridgeton Station. It's not as crowded as the spotter's lot and is much better for shooting approaches on 12R.
To get there from the spotter's lot at Lindbergh and Missouri Bottom Rd, turn left (north) on Lindbergh until you get to the first stop light (usually flashing yellow) in front of Bill Thomas Camper Sales. Turn right. You will immediately reach a T-intersection. Turn right and follow the curve around until you see the wide shoulder on the right.
A word of caution. Do not park on the east (airport) side of the road. The Airport Police will ask you to leave. Also, up until TW 800 the police didn't care if you walked up to the fence so long as you were parked on the shoulder across the street. Since July of '96 they've been chasing people away. Since they usually drive by only once or twice an hour, you can probably get off a few quick shots of 12R departures.
If this is an ANG weekend you're in for a treat if 12R is active. When the F-15s run up you'll be close enough to feel your car windows vibrate.
If the 30s are active, dip into your life savings and buy a beer at the 94th. The patio there is great for 30R activity and it's located right next door to Midcoast Aviation. (STL's biggest FBO) The Lears and Gulfstreams will taxi right in front of you.
Hope this helps.