It's very important to lay down the expectations up front with our clients (hence the purpose for the engagement letter included in the program). Make it very clear what the fees are, when you expect to get paid, what happens if they don't pay (or on time). For example, your new client should already have the expectation of making a payment at your first meeting. Then, depending on your meeting schedule, they should know exactly when the next payment is due. Make it very clear.
Finally, on a related note, let them know what your meeting cancellation policy is: how much notice you need, what they still have to pay for. My recommendation is to always get next month's meetings on the calendar and let them know they pay the fees even if they cancel those meetings. That will maximize your time with them, which will in turn maximize the results.