Reminding a professor for Reference Request

Hello everyone, this is my first post here.

The situation is like this. My professor has promised me to act as my referee for my LLM application. I've sent him a "gentle" reminder for about two times now and he either ignored (no reply) my email or said he will look into this soon.

As the deadline for the LLM application is approaching, I am feeling more and more desparate to have my professor to write the reference letter. I, however, do not want to sound pushy in the next email "reminding" him that.

I try to be firm (but not pushy), and understandable (that he's indeed quite busy or he's just too fogetful).

The problem is that I've run out of things to say in this (next) email.
I am trying to write in a different style (but with the same tone - firm, understandable - requesting reference). I don't want the same words appearing over and over again in the next couple of emails reminding my professor.

Can someone please guide me in writing a reminding-letter?

Thank you very much.