Hi Davy,
I mail all residential customers 11 months after the cleaning, unless they have hoardes of kids in the house!!six months in this case.
For an offer i generally hold the price the same as the previous years, as you know it gets dearer every year to run your business.
Also after every job i send the customer a thank you letter and occasionally i send a discount voucher for 15/20% off the next clean, but be careful with this option and make sure you put an expiry date on the voucher so that it ends in six months as i got caught short before and sent the reminder, hoeld the price as the previous year as the offer and then the customer gave the discount voucher also so i was down a few bob!
You can also highlight on a voucher 'NOT TO BE USED IN CONJUNCTION WITH ANY OTHER OFFER' because holding the original price is an offer.
Hope this helps
Cheers
Scott