Participate in LinuxChix. Respect its goals. Encourage women who are smarter than they think they are. Applaud women who know exactly how smart they are and are proud of it. If you're male, let women speak for themselves; don't 'help' them; work with them. They might not need your help, after all.
Tell women interested in Linux (or UNIX) about LinuxChix.
If you are in a hiring role, actively try to place job advertisements in places where they will reach interested women, for example email@example.com.
If you are involved in a Free Software project, attempt to find some women interested in participating in the project and offer to mentor them (depending on their interests, introduce them to the mailing lists, offer to help them understand the dirty bits of your project's code, teach them how to make use of your projects bug reporting system, or ask them to document some part of the project). You can do this if you are not yourself the author or a developer - if you are part of the project's community then you can help others join. Post an offer to LinuxChix every now and then - "I'm involved in X project - interested?"
If you're involved in other computing or software groups, think about how woman-friendly they are. Do you have a culture of needing to prove your technical prowess before you earn 'respect'? Do you have a culture of insulting newbies or outsiders? Do you have elements of actively unfriendly attitudes to women - for example, your documentation refers to the user and "his girlfriend" or your IRC channel spends a lot of time talking about how they hate women? Do women who join mysteriously vanish in a short period of time?
You may not be able to actively challenge your group if this is the case. If you command considerable respect in the group you might be able to influence others by your own behaviour. Be polite to women who join. Re-write the documentation so that it is gender neutral. Tell the overly-amorous young geeks to get a hold of their hormones already. If you are discussing a woman who is part of your group, or having a discussion with her, stop the unnecessary references to her gender, appearance, relationships, sexuality or other utterly non-technical things.
Remember that often overt sexism is a group behaviour, used as a bonding activity. It will often be led and controlled by one or two ringleaders. You might be successful if you attempt to speak privately to some of the participants who are not the ringleaders. There are two advantages to this - you are taking them away from the group which validates their behaviour, and you are personally calling them to task for their behaviour and not letting them hide in the crowd.
If not, you might, if possible, want to consider starting a new group that isn't so hostile, or consider what you are doing there in the first place. One of the easiest ways to make sure a new group is women-friendly is to have women there. Lots of us, preferably, and at all levels, from administration down to occasional user or attendee.
If you are in a group that is friendly to women, but low in numbers, try to get more women to join. Lots of us are looking for more places where we can be both geeky and safe. If nothing else, tell LinuxChix about it.