Ok, lets see here. Much has been covered here, but responses have been slightly vague. Lets get REAL specific here. Checklist.

Plugs - Brand? - Every 30k
Wires - Brand? - Every 30k
Oxygen Sensor - Brand? - Every 30k
PCV - Every 30k
Coil Check
Fuel Filter - Every 20k
Clean MAF
Air Filter - Every 12k

Done all those? If you have...and not used ONE Bosch part...then lets continue...

Vacuum Leaks, check all hoses, connections, etc. Check for cracking.
If none are found, check the Vaccum hose connection on top of the intake. I had one here, took it off and made a gasket for it from gasket material.

If you're STILL having the problem...

Clean TB
Clean IAC

Now, if that doesn't fix the mileage problem...
Remove both front wheels and check for spacing between caliper and rotor by looking straight down on the caliper. Good way to check is to slide some paper in there if you can't tell. If there is space on the inboard pad, but not the outboard, remove the caliper and grease the slider pins. If both are constantly contacting the rotor, you either have a collapsed rubber brake line, or your caliper is no good (less common, but possible)

If you can check off on EVERY one of those things, that'd be excellent to move on in the diagnostic process.

Gotta disagree with the service interval on those two items:
Oxygen sensor should be good for 60K unless contaminated from coolant (or someone dropped it!).
Fuel filter should be good for more like 40-50K unless you have a fuel quality problem.

It won't hurt anything to change them sooner, but it really should not be necessary.

And, there is a benefit in changing the fuel filter more frequently. If you change it more often, you will be able to change it without reconstructing fuel lines in front and behind it.

Thanks. I',ve replaced most of those things with napa parts, and will now swap in the Buicks coils to check that.

It was the coils. Thx.