How can you concentrate while working on a computer & not be distracted by the internet?

I always have problems when I have work that needs to be done on a computer (typing long notes, since my hand cramps up writing it, or doing essays and research topics). My self-control went down the drain years ago, so I always find myself checking my email, my Twitter, my RSS subscriptions, forums I visit regularly (and Fluther looks to be a serious contender, too).

I need methods that will help me, either psychologically or actual technical solutions (like blocking my favourite sites for certain times of the day).