As an ex-woman-from-tech (but not a technologist), speaking solely from my own experience and observations, these things would help:

• Don’t use the “I‘m not sexist and I try to set a good example” clause as an excuse not to engage with sexism or take action on a constant basis. (Obviously you’re not, or you wouldn’t have asked the question, but this is at the top of the list.) Sexism is everyone’s problem.
• Never, ever, EVER say “I only want to work with the best. People should be hired on their merits!” This automatically implies there aren’t more women/minorities in tech because those people aren’t any good at it. It’s a myth put about by elitist white men who like being on top. Promote different ways of thinking about team work and actively challenge those who propagate this bullshit. If you’re in a position to hire or promote, hire or promote accordingly.
• Call out every single act of sexism you see or overhear. Ever. Strenuously.
• If a woman or person from a minority group is trying to initiate a change in your workplace, get the hell on board. Then, encourage everyone else to. Stonewalling ideas (or just ignoring them) might seem justifiable for a million “business” reasons, but from what I’ve observed, it’s simply a resistance to the change that more women and minorities in tech herald.
• If someone at your work confides harassment to you, don’t (only) offer to punch the offender in the face. Firstly, do whatever you can do to support that person on a personal and professional level, knowing full well that they will probably never report the harassment for fear over their career. Also, it would be interesting to see what would happen if men advocated for victims of harassment—without identifying them—and against the harassers. When I was harassed I would have loved for the person I told (who was a personal friend as well as a highly respected member of the core dev team) to raise the fact that he’d heard that my harasser was harassing staff either with HR or someone in management, because those people needed to know, and I was not about to tell them (I left the company instead). So perhaps offer that to the person who’s confided in you and see if, through doing that, you can either initiate more watchfulness on the part of the people running the business and/or actively change the way gender, power and its abuses, and so on are handled. (NB Other women may well disagree with this point.)

As an aside, I think the “you have to report it or it didn’t happen” storyline about harassment is getting the industry absolutely nowhere. If you’ve never been harassed, you probably don’t realise how diminishing it is. Sexual harassment in the workplace reduces the victim to an object. A body. It strips away all their unique talents, skills, and hard-earned expertise, and reinforces all the traditionalist, misogynistic stereotypes about women that we as a species struggle daily to overcome. As well as making the harassed feel small, insignificant and powerless, it makes them intensely self-conscious. So for people and businesses to say “we can’t act if no one’s willing to come forward” only perpetuates the present situation. It serves absolutely no one — not staff, not share holders/investors, not owners, not customers—except the harassers.

If I had staff and I got wind that someone felt harassed by person X, I’d tell person X so, outright, regardless. If someone’s behaviour is being taken as harassment, or as sexist or otherwise biased, they need to know about it so that they can consider and change it. Simple.