For me it is often staying on top of billing. This sounds counterintuitive but since I am always focused on the next project I often forget to chase down payment...
I understand that it's hard to stay on top of your payments, it can be quite a tedious task... For me however it's finding Clients! Really struggling with that
Just keep pushing with that one. The more good work you do the more that comes back to you. I swear. You will be at the point that word of mouth is all you need.
you can simply create a spread sheet with all you clients , mention the start date and the due a mounts on each of them , so you can focus more on getting new clients.