When first starting out you have to be very aggressive in advertising. Here are some things you can do:
1. Postcards! One way to set yourself apart from other cheap lawn guys is to make double sided glossy color postcards and put them in the newspaper boxes. People are less likely to throw these away. I got 2500 of them for $140. Just drive around and put them in the newspaper boxes.
2. Craigslist add. Find someone that can create a nice craiglist add for you and keep it posted on the 1st page as much as possible. Repost it every couple of days.
3. Website. Today websites in my opinion are the best way to get good customers besides word of mouth. In order for them to find you, you need someone who is good at SEO (search engine optimization) to get you listed on google, yahoo and such when people search for lawn care companies. Investing in a quality website will get you very far.