I sold off my alto when it started this behavior, my case was even worse. I used to get avg of 8-9kmpl in city, it mostly ran in city.
I lived with it for a year and tried all possible solutions, even went for an engine ovehaul even though car had only touched 60k kms.
Then I bought a nano for city abuse, and it too gives a max 15 kmpl.
Dont go fo engine overhaul etc. just do basic servicing such as air filter cleaning spark plugs, fuel filter etc and drive sedately. Though i never learnt that way.