When we talk to people that have wireless networks, these are the first three things we ask them from a security perspective:
1. Is your network encrypted? If so, what kind of encryption is it using? (WEP, WPA, WPA2, etc.)
Tip: Unless you're using WPA2 encryption or better, your network is as good as wide open.
2. Are you broadcasting the name of your wireless network (SSID)? If so, does it identify your company in any way?
Tip: Many companies have networks that are named after the company. It's logical- but it also makes it simple for a hacker to identify your company's network! If you have to make your wireless network name(s) public, make sure to use names that cannot be traced to your company.
3. Do non-employees ever use your network? If so, do you have a dedicated, secure guest network?
Tip: If you need to give outsiders access to a wireless network, you should create a separate guest network. The guest network should be totally separate from your internal network- and also encrypted using WPA2 or better encryption (with different password than your regular network, of course!). Bottom line: never let a stranger onto your network. Even if there's no ill intent, a single virus from an unsuspecting guest's laptop can devastate your company if it gets inside.