The best advertisement is a job well done. If your customer is happy, they will recommend you to others. Additionally, neighboring property owners and passers-by who see you doing a good job will be inclined to talk with you about doing their driveway and sidewalk too.
If you're going to shovel for people, consider having icemelt available. You could spread it on the driveways and sidewalks for an additional charge. If your customer chooses to have you salt their driveway, not only will they have a safer environment to live around (no ice), but your work will look even better than it otherwise would.