So you are saying you would mow someone's lawn for free. If they liked it they would continue and if they didn't it was just a free mow?
I would not suggest you do that. That could lead to a lot of free mows and no customers. Instead, the time you spend mowing those free lawns, and the money on gas, machine wear, etc. you can spend that on going door to door and getting customers. the money you spend on gas for the free mowing u could use to have business cards made or fliers you can hand to the potential customer.
Instead, analyze why you don't have as many customers and change some of the things you did in order to attract more potential customers.
Try doing the following:
Have business cards made out with your info on the front of the card and a coupon on the back (coupon could be 10% off 1st month, or final mow free with annual agreement but put an expiration date if you don't want to use the same offer in the future years). Then pick an area you would like to do business in. (generally the closer to home the better, less travel time). take those cards and place them in as many place as you can...example: gas stations, restaurants, etc. also take some cards and go door to door and hand them to your potential customers. let them know about the coupon on the back.