How can I protect my site from being leeched?

[mateo]

I am using the \[code\]header\[/code\] function of PHPto send the file to the browser with some small code. Its work welland I have it so that if any one requests it with a referer other than my siteit redirects to a page first.Unfortunately it's not working with the internet download manager.What I want to know is how the rabidshare and 4shared sites do this.