You can't force people to turn up to the site, but what you can do is make sure that the site is welcoming to new visitors of all kinds.
For example, make sure that things like sexist names and avatars (pictures of scantily clad women for example) are known to be inappropriate and will be removed when found. Such things are already against the Stack Exchange terms of service, but it would be worth reminding people - particularly if you have recently had some issues in this area (NOTE: I'm not saying you have, just that you should take preventative action).
Encourage people to edit out any inappropriate language from posts. Again, this shouldn't be there - after all you are running a professional site for professional people, but sometimes people slip up.
Encourage people to use comments wisely and discourage the use of any form of name-calling etc (things like "you idiot" etc.) It can be very discouraging for anyone to be greeted by a slew of negative comments when they first show up and ask a question or post an answer.
I know that sounds like I'm stating the obvious but if anyone comes to the site and their first impression is that it's a closed club and they're not welcome then that's certain to ensure that they don't come back.
If you do run "recruitment drives" (for want of a better term), have people actively looking at the site for new posts from new users to make sure that they get an encouraging welcome. This doesn't mean you should drop your standards - an off topic question is still an off topic question - but make sure that criticism is constructive and helpful. Don't just say "this question isn't welcome here". Look for the good question that might be buried in a badly formatted post. Show by example what a good question and good answer look like, and so on.