We send out a letter at the beginning of every year...but it is for them to call us to say they want to use again, not to call is if they don't. I find it's better to speak with your customer beforehand. You know, a little customer service. Hello Mr. Doe how was your winter? Are you looking forward to the warmer weather? Just seems better. IMHO, I think if you send something around saying we will renew unless you call is asking for trouble...and I'm talking from a legal standpoint. Not sure you would have a leg to stand on.
As far as the actual letter...I like to use a standard letter but personalize it a little. Something along the lines of we serviced you last year for this amount. To utilize us again it will be X amount. Perhaps you can add things in like we noticed their was a bare spot in your yard in a high traffic area if you'd like we could overseed it for X amount. I'm not suggesting to BS them, be honest. But at the same time let them know you are thinking about the care of their property and what needs to be done/could be done. Just my two cents worth.