They need to know when they are expected to make payment, and what the consequences are if they don't. This can be achieved through clear communication from the outset, and by sending invoices on time.
If your customers don't pay on time, Xero has a clever automated email option that can be used to nudge them. If they still don't pay, we can make those tedious phone calls and even advise on next steps if no payment is forthcoming. However, it's always best to avoid this situation, if possible, by being clear about payment terms and expectations from the beginning. We can help with that too.