I have found that the key to success here is flexibility. I have customers whose frequencies range from weekly, fortnightly, 3, 4, 5, 6, 8 or 12 weekly. It may sound a nightmare, and at first, as you can imagine, I didn't know where I was. But as I have got into the swing of things, it actually works well for me, as I now have a very simple system of keeping track of when cleans are due. Each customer gets serviced according to their individual requirements, as opposed to doing every house I have in the same street at the same time. All of my customers in my town can be reached within 10 minutes of my home, so it really isn't a problem for me to work this way.
I do have a couple of days work in another town about 20 miles away, which I have to be more rigid on, but as my customers there know that I have to travel to service them, they then either have to have them cleaned when I turn up, or wait until next time.
John