Ensure file and printing sharing is turned off or limited and check for other file sharing software (ie – Morpheus, Kazaa, Audiogalaxy, etc.). Many of these programs share files without your knowledge and may cause unnecessary usage. Even if you are not actively using these applications, they may still be serving out files to the entire Internet.
Chat programs (ie – ICQ, Messenger, IRC, etc.) can sometimes cause problems when they are configured to accept downloads.
Restrict access to your computer to only trusted people and consider using a password to prevent others from using your computer without your permission. Treat your computer much like you would your cell phone or long distance account.
Check downloaded programs and files with an up to date virus scanner. You should also regularly check for updates and security patches for your operating system, Windows users can visit the Microsoft Update page and Macintosh users can go to Apple's Security Update page. For other operating systems check the vendor’s homepage.
Consider downloading a personal firewall program. Personal firewalls help keep track of traffic and connections to your computer.
Remember that streaming media also counts: just listening to a radio station impacts consumption. A high quality MP3 audio stream can also use about a megabyte per minute while high quality video streams can use two megabytes per minute.