[Iflylow]

You didn't say which engine you have, but it doesn't make a large difference. The biggest factor in gas mileage is how heavy your right foot is. Slow starts, coasting to stops, easy acceleration, combining errands into one trip...all help.
After that, I'd check tire pressures every week or two. Emptying the trunk/back seat of unneeded weight will help also.
You should just leave it in Overdirve all the time. By driving in 3, you are preventing it from shifting to the higher, more efficient gear when conditions allow.
I get 23-25 combined town/highway mileage in my 3.8 2000 Impala. From what I've read, your mileage sounds pretty normal.
The numbers on the window sticker are obtained by driving a specific test cycle, and don't even come close to real world driving conditions. The test drive is never over 55mph, no air conditioner use, no stop and go traffic, no stuff in the trunk etc...those numbers mean very little, and you will probably never bea ble to match them in the real world, sorry.