I will fertilize their lawn 4 times a year using Scotts fertilizer I bought the 4 step program from Scott's to do this. I was able to increase from 12 accounts to about 40 accounts this year.
It seems like you are doing a great job so far getting the word out.
Can you tell us a little more about this offer? Do you offer free fertilization if they sign up with an annual lawn care agreement?
As far as ranking higher with your website, have you thought about the quick and easy way to do it, buy online ads on the search engines?
Otherwise if not that, why not get a blog on your website and start writing articles with key words that your customers will be looking for.