For me, anyway, the answer is tracking time.
If I want to do something, say, work on a project, I set a timer for 30 minutes and start working on the project. My rule for myself is that for 30 minutes, I have to work on the project, even if I'm banging my head against the wall and not making any progress. After 30 minutes, I either keep working (if I'm in the zone), or stop, re-evaluate and repeat - but never stop before 30 minutes is up. At first, this was painful, but now it's pretty easy to get into the flow. I do this also with things I really don't enjoy, like dishes and housework.
While I think that the rule itself is arbitrary, I think what it did was help me to build discipline, which was immensely important.
Then, video games and other entertainment go at the end of the day, where I can enjoy it guilt free - and importantly, where it can't overlap with work. One nice thing is that I don't feel guilty having fun anymore. Of course, I'm not so strict, sometimes I wake up on a Sunday morning and play games for three hours, but generally I try to stick to this plan.
As other people have said, you need to separate what you need to do over what you want to do.
Another technique that I use is to separate work space from play space. I'm not going to go and play Starcraft II (my chosen drug) in a Starbucks, so if I'm feeling particularly distracted, I'll go there to do work. I live in NYC so I don't have the luxury of having a spare room to use as an office, but if I did I'd seriously consider setting up an office separate from my main desk (ie. have a video game computer and a serious computer).
Finally, start slow. Lots of people look for big overnight change, don't get it, and revert to old habits. Instead, start slowly. Do one of these 30 minute sprints each week day, and two each weekend day.