I just wrote a tutorial post on our web design blog that outlines how you can use an .htaccess file as an additional layer of protection for your web site hosted on a Linux/Apache server.
No web site is completely safe from hackers. If someone wants in bad enough, they will keep prodding the defenses until they find a weakness. However, if you correctly use the .htaccess file on your site, you will stand a better chance of blocking most attacks.