We'll also assume that you're driving like a grandma, and not doing burnouts, drag racing other Bonnevilles, and the like.
I wouldn't count out people stealing gas, but if it'* continuously bad mileage, you can probably assume that'* not it (unless one of your co-workers is always popping breath mints at the same time every day).
As far as tune-up parts:
- Spark plugs are rated at between 30,000 miles (coppers) and 100,000 miles (other metals).
- Spark plug wires are generally rated for 30,000 miles: check the box.
- O2 sensors will last forever, but lose effectiveness after 60,000 miles. (I got an extra 40 miles per tank by replacing my upstream one.)
- PCV valves are cheap enough to replace, but as long as they work (rattle), that'* fine.
Also, check your tire pressure and alignment. Refill and realign as needed.