Like wired, you can make wireless as secure as you want and someone can still get through.

The problem is you open up one more way to reach your network, by tapping into the wireless.

However, there are many steps you can take and it all depends on how much you want to spend.

The most basic wireless routers let you take the step of allowing only approved MAC addresses but then you have to know the address of every computer on your network but it will work.

You can also invest a lot more and get a router with much more control you so can limit the reach and the "shape" of the wireless (you can limit to a cone in one direction for example) that will keep it more limited to your office only and cut the chances of someone just walking by and tapping in. You can also go up to passwords that require 20+ characters that make it pretty hard to break into.

But pretty much all wireless routers carry basic firewalls that you can implement and, as always, it's up to the users on the network to not click on a suspect e-mail or link and invite in someone unwanted.