If you're relying on a Web host or ISP, be sure to choose a vendor who is security savvy. Compare their offerings to those of other companies. When an employee leaves a company, remove the employee's network access immediately. When asked to evaluate the internal security posture of networks, the Cisco Security Consulting team finds vulnerabilities in almost every network tested. Just as you ask departing employees to turn in their keys to the front door, you should take away their key to the network when they leave.
Disgruntled employees are the greatest threat to any systems' security.
If you allow people to work at home, provide a secure, centrally managed server for remote traffic. Telecommuting increases worker satisfaction and productivity. But it also presents a security challenge. Update your Web server software regularly. Stay on top of security updates and patches.