The Zetec, even in the Focus, can get quite thirsty in combined and/or city driving. If you have an automatic, you will notice a lot of gas use.
That being said, just make sure you always replace your filters and fluids at the recommended intervals. You may also want to look into cleaning the fuel injectors/system as well.
One big culprit for increased gas use is underinflated tires. Make sure they are inflated, as indicated in the owner's manual and/or the decal on one of the door pillars.
For spark plugs, I recommend sticking with Motorcraft only because the Focus folks have noticed some problems going aftermarket in their Zetec engines. Although Ford advertises its platinum tipped plugs to be tune-up free until 160,000 km, normal wear and tear, bad gas, poor quality, etc. can result in a plug going sooner.