Wait until you've been going a few years and you have to do hundreds and hundreds of the bloody things every few months.
Personalise the letter if you have a mail merge database Dear Mrs Smith not just Dear Customer
Put a headline in the letter "Special Christmas Offer" for example
Thank them for the previous business, give them good reasons for cleaning again, appearance, keep fabrics looking good, remind them how good it looked, how convenient you are, offers are usually good depending how busy or not you are!
Put a ps with a cut off date reminder for any offer and your phone number.
Put a return address on the envelope so the Post Office can return those that have moved, then send a general letter later to The Occupier.
Send them First Class, the amount saved is neither here nor there.
The return may only be 4-5% tops but they tend to spend, the last one I did brought about 8 times the amount spent.
A few quick pointers hope it helps.