Use the postcard method - I can't say it works every time but it is super effective.
Go to your post office or use the services on usps.com to get yourself set to send out a bunch of postcards. I used to prefer the actual handwritten card as it looks better to the neighborhood you are targeting as a real person is mailing it.
Hop in your car with a tape recorder and drive slowly up and down the streets you want to target.
As you pass a house, say the street address and the amount you would charge for mowing their lawn.
When you get home either start writing or go online and get your post cards ready to mail.
Address each card and on the back write something along the lines of:
Hi Neighbor,
My name is drin101 and I only live a few blocks from you. I'm starting a small lawn service and would love to have you as a client and can take care of your lawn for only <insert amount> each time I come by. You will have more time to do the things you really want to do as opposed to additional chores and you will be comfortable knowing that a neighbor is making sure your home looks great.
Give me a call at <insert phone number> at your convenience and I'll offer you a discount on the first mow!
Thanks!
Your neighbor, drin101