[orange4boy]

Have you checked tire pressure? Lower rolling resistance can be a big MPG improvement.
A dead or malfunctioning oxygen sensor can mess with your mpg. Your car could be running in open loop mode meaning no computer control.
Next, extra weight removal,
Battery holding a charge? A dying battery can cost big time. If you run short trips with a lot of alternator load, your battery may be constantly charging sucking amps. Trickle charge overnight. Turn off electrical loads.
A block heater can help a lot on short trips even in California getting you up to operating temp much faster.
Rear wheel skirts are relatively easy and effective mpg help.
Lower your idle speed. Some times a lazy mechanic will set it by ear to save looking for the stock rpm. If you sit in traffic a lot and put your car in neutral or turn it off when stopped.
Check ignition timing. If it's retarded can cost mpg

Just some stuff I didn't see posted yet that worked for my van.

Keep trying, you will get there!