Until recently, I was hopelessly addicted to the internet, and it was affecting my productivity at work (Nope my manager did not point this out in the yearly appraisal - it was a self realization :) )
I'm still addicted to the internet now, though on a much smaller scale. My two cents worth of tips-
1. LeechBlock
LeechBlock is a Firefox add-on that lets you set timeout policies (i.e what to block, when to block, how to block) for different websites. Start with a daily goal and progressively cut down the total time spent online.
2. Cut down on friend feeds
If you see your Facebook friend list (assuming you spend considerable time on Facebook), many of them don't belong to the "close" circle and clearly you have little interest in their daily status updates. Disable their status updates from your feeds, and you suddenly find Facebook less noisy. Of course, they're still on your friend list, you can always find out what they are up to if you want to.
3. Check Facebook/Email only when updated
People generally don't check their mobile phones every minute to see if they received a new SMS - new messages gets notified by a beep or buzz. On similar lines, you can enable POP3/IMAP access on your mobile phone and you will be notified of any new emails- you need not hit refresh every 3 minutes t o realize there are no new emails. Or you can use Webmail Notifier add-on in Firefox which pops-up and makes a notification sound when a new e-mail arrives, saving you the trouble of hitting 'refresh' and the frustration that follows.
You can install Yonoo sidebar which notifies you of status updates from your Facebook/Twitter accounts, all in one place. Yonoo sidebar when used with tip#2 above can be very effective- getting notified about the status updates/tweets of closest friends as and when they happen, at the same time preventing you nuking the refresh button every 3 minutes.
4. Use an RSS reader
You can use an RSS reader (like Google reader) to deliver feeds from your favorite blogs and sites all in one place, so that you don't have to frequent those sites.
5. Garble your Facebook password
If checking Facebook becomes obsessive-compulsive and if nothing above works, this is the final step - garble your Facebook password to something like $&;*%*W and store it in a text file somewhere on your home PC (so that it's inaccessible at work). Believe me, after a dozen times you would be to lazy to recover this 'forgotten' password while at work!
