How do you create a female-friendly work environment at a startup?

If you foster an environment of professionalism and ethics, the environment should be friendly for all types. If you try to develop an environment that is pro one group vs. all others you are headed for trouble.

Anonymous;

My first Anonymous reply! I'm very excited.
1) Hire women. Greenspan commented (back when he ran Townsend Greenspan) that he had a built in bias for hiring women...and he did so aggressively. I do so as well, on the theory that if the market undervalues them, you'll be better off. As to why that creates a better environment; one woman amongst 30 bro-grammers does not strike me as a recipe for a good environment. So the plural case seems to have face validity. Has it mattered historically? Not that I can tell...but I'm not in their shoes.
2) Be alert, and aggressive, when it comes to harassment. This doesn't mean single people out for protection. What I mean is be pro-active about education and make sure there is a means to report issues. If issues appear, have a process and follow it.
3) Hours. Double-edged sword. Be flexible. A good friend (single mom) got picked up recently by a very large and well-regarded tech company in the area. They have a reputation for long, awful hours...not the worst, but a reputation. A couple small tech start-ups wanted her. The large company won; She shows up at 9, leaves at 4:30, has flex for school and illness and such, and makes it up on the margins. I personally view her as singularly brilliant, and she is quite well regarded. The big company got what it wanted (her skills) by giving what she needed (lots of flexibility). There is bias in my voice here....I'm a single dad, so I have the same problems, and have to have the same negotiations. However, I still believe that this is a problem more common for women (both single and in partnerships).
4) Inclusion. My grad school practiced a 15 second rule. Theory: women are often (not always) less aggressive about responding. Watch for the delayed response and be proactive in including their voice; actively solicit their input and pull them in. This helps out the introverts as well.

That's the best I have...I'm a relative amateur here, and will look forward to other voices.

You talk about creating an environment as opposed to creating a hiring policy aimed at recruiting(more) females. The answer is to respect the views of all employees equally, and give credit where credit is due. Any attempt to create a female bias will likely create a hostile environment among the male employees.