I'd suggest putting a "code of conduct" or "guidelines for participation" or something somewhere moderately prominent on your site -- perhaps on the front page and/or in the sidebar menu. In it, explain what is and is not OK. If you say "sexist comments" people won't know exactly what that means, so you might want to say something like:
"Comments that denigrate or ridicule women or any other minority group, or which suggest that members of that group are not or cannot be members of our community, are not permitted on this wiki."
Wrt the thing about inclusive language (him/her and the like), I believe it's important, but there are a few ways to go about it. These days "him/her" is not unusual and most people will just skim right past it -- it appears in eg. forms you fill in all the time. People could also say "they" as a non-gender-specific singular; though people will complain that it is poor grammar, it has been used in English ever since Shakespeare. Another option is to rework the way sentences are framed so that it avoids the pronoun (though this can lead to the passive voice, which some people dislike). A final way is simply to switch it up a bit: use different pronouns in different places, with an approximately 50/50 ratio.
More frustrating to me personally is the sort of language that that says things like "we all know computer geeks love pictures of sexy women" which, while not using the male pronoun, definitely sets up the assumption that "computer geeks" are male (and heterosexual). An example from this comment thread is the "women hate geeks" comment a while back: setting up a false dichotomy with "women" on one side and "geeks" on the other as if there were no such thing as women geeks. To me, this stuff is more insidious when it comes to making us feel invisible and unwanted.
The final thing is to take complaints about sexist comments/jokes/pronouns/whatever seriously. What you need to say is something like, "If women say that something said here denigrates or excludes them, we will believe them and take them seriously, and change it." In a difference of opinion between a man/men and a woman/women over whether something is sexist or not, you need to give the woman the benefit of the doubt. Otherwise, what you are saying is, "We don't allow sexist language except when we really want to," and that is not a strong or welcoming message.
Anyway! I am rambling.
I am thinking of putting together a wiki page on the Geek Feminism Wiki that lists projects that have a code of conduct that prohibits sexist behaviour. If I create it, and after you've set up your CoC, I will add your project to the list.