Of course, once I do start working on something, I get in the zone and completely lose track of time. Once in the zone, I'll work at a task until I've completed it. Overcoming the "inertia" is the difficult part.
This is what I've done to combat my situation: I've started keeping a TODO list. I've also cleared my desk of unneeded items and removed useless feeds from my RSS reader. I'm getting into the habit of putting my MacBook to sleep when I'm not working. That way, I can't wander off to Digg or the WTF subreddit. I feel I'm about 2x more productive now, but I'm still far from the stage where I can spend more than 50% of my computer time doing productive tasks.
Tips? Links? Blogs? Also, I love anecdotes :)