[michael_fine]

Two ways that work for me:

One, don't try to reason it out. Don't make every chance you have to slack off be a constant mental battle of willpower. This is why eliminating distractions works so well. Also, just make it a habit. This is the harder part, but try something like the pomodoro method, where you work for 25 minutes, and take a 5 minute break. No distractions at all during that 25 minute period. Once that becomes a habit, it gets much easier.

Conversely, if you do find yourself wondering why your doing what your doing, or you have to convince yourself, just think of what you want to accomplish. Is it going to the gym, or working on a side project, or reading more? Just think of the first step. The first step of going to the gym is just getting in the door. Getting in the door is not very hard, but how can you possibly get fit without getting to the door? Whenever you lack motivation, just ask if you've completed step one (http://www.scotthyoung.com/blog/2011/07/18/step-one/)