There are many ways to automate reminders. This could be via letters or emails to the client, or reminders in your diary or calendar for yourself to follow-up.
Consider having a phone conversation, rather than just sending letters and emails.
Using the telephone
Firstly, if they are not answering, or not prepared to talk to you, this may be a sign that their business is not doing well, or that your bill is in danger of not being paid on time (or at all).
Secondly, when you do get to speak to them, there will be many more cues as to whether they are able to pay. These include the actual words they use, but also the tone of what they are saying and how they are saying it.
If you start to get the feeling they are not telling the whole truth, or perhaps they are somewhat uncertain about what the future holds, then you may have an early warning sign that your bill may not get paid.
Finally, if they are indicating they will have trouble paying, or have already missed the payment deadline, consider making a new arrangement on the spot. Be prepared to make a suggestion, see how they react, and be open to a proposal from them. If you can make a new plan, follow-up with something in writing that confirms the discussion.