In order to politely follow up with a client, it is important to keep in mind the fact that you are aware of their busy schedule and value their time. As such, it is key to be concise yet polite when reaching out for a second time. This could be done by referring back to your previous email in which you requested to have some sort of response, and expressing your gratitude if they have already responded.
If they have not yet responded, then it is important to express understanding while also politely reminding them of the request in question. Be sure to remain courteous and polite throughout the process while also keeping your message short and easy to read.