I've taken a slightly different approach. I'm trying to get more work out of my existing customers.
Make sure that your customers are aware of all the services you provide and even suggest things you see that are in need of being done.
I had one mowing customer ask me to trim her Boxwood hedges on each side of her front door. It was a good opportunity for me to mention to her that all of her shrubs really needed pruning...seeing them every day she really didn't realize that were overgrown and out of shape. Some had never been trimmed.