It surely depends on what the OP is providing. If I was a consultant and the company stopped paying, I would obviously stop working for them on the next day. Otherwise, you could start charging weekly or daily instead of monthly. But then again, if they're not paying...

If I was providing a software or utility, the second you stop paying you are out. This actually happened to me in Japan because my mobile provider stopped sending paper invoices so I forgot to pay. I got cut off without any warning and as soon as I paid I got my access back.