Above and beyond any practical advice, the most effective way to make your workplace mental health-friendly is to talk about it. Encourage everyday conversations about everyone’s mental health. Make it OK to talk about and everything else will follow.
Having plans in place to support mental health and offering them to everyone as a matter of course can be helpful. Here at Rethink Mental Illness we use a tool called a wellness & recovery action plan, or Wrap. This is a document drawn up by the person in question, based on what they know about their own experiences. Sometimes this type of document is referred to as an advance statement.