Best thing to do at this stage of your business IMO is to get some NICE fliers made and campaign to the 4-5 neighbors of the current yards you mow. Every time you cut, go to the house on either side, and the 2 or 3 across the street. If you are in a cul de sac (sp) hit up the whole thing. Oh and dont just put the flyer out, actually knock on there door and say something simple like "Hey I was cutting your neighbor's yard and wanted to take the opportunity to talk to you while I was here"
This is what I did when I was pressure washing and it had great results.