With my regular custys on domestic cleaning I have most of them on a standing order (much easier to set up).
The custy sets up the SO to pay the same amount every month.
I work it out on a 48 week year, so those months that have 5 weeks in or holidays make it a 3 or even 2 week month then the custy is either in credit or debit and this shows on the invoice each month.
Its like your gas or electricity if you on DD, you pays the same amount even tho usage varies thro the year.
Standing orders are easy to set up - done by the client. and by keeping it the same amount every month it helps the client budget - its a good selling point, used it many times.
Direct Debits I think are going to be more difficult for a small business as you have an input. DD can be varied each month so the custy always pays whats on the invoice.