I always wondered what people hearing me stammer think about me, I just hope they're as kind as you are. I think not completing their sentences, being dismissive/impatient is a good start. And please don't scrunch your face as if you are trying to comprehend what they're saying as if they're speaking an alien language. I know our speech might be difficult to understand some times, but I really hate when my prof does it. It makes me not want to engage with them anymore.