We renew every year (letter/form). Letter goes out january 1st. It is time consuming and expensive, however, the client feels like more than just a number. It also gives us the opportunity to directly market additional services and update our records with new phone numbers and emails. We also enclose a business card for referrals. Annual renewal letters work well for us, we always add new clients as well as sell substantial aerations etc!
When i find myself in times of trouble, mother mary comes to me, speaking words of wisdom let it be.