How to minimise DNAs as a free service?

Our organisation provides therapeutic support and counselling for free. However, we experience a lot of service users not attending their sessions (DNAs) which is putting unnecessary strain on staff time and resources when we are already struggling with understaffing and the current funding environment.

We were wondering how other organisations are mitigating and minimising their DNAs as obviously we cannot charge clients for non-attendance. Do you have messaging on your website/social media? Posters in waiting rooms highlighting the cost? 

Any advice is much appreciated.