"Let Go", by Randall Munroe, licensed under Creative Commons Attribution-NonCommercial 2.5 License
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After years of trying various methods, I broke this habit by pitting my impatience against my laziness. I decoupled the action and the neurological reward by setting up a simple 30-second delay I had to wait through, in which I couldn't do anything else, before any new page or chat client would load (and only allowed one to run at once). The urge to check all those sites magically vanished--and my 'productive' computer use was unaffected.