A new method:

Instead of trying to find motivation, change your criteria for doing something. Say when you look at an activity your criterion is how happy you'll be immediately, at the start of the activity. This leads to slacking off. Who wants to do homework, which is immediately tiresome, when one could do computer games instead and be immediately captivated? So, change the criterion. How will you feel fifteen minutes into the activity, or, how will you feel fifteen minutes after the activity is completed? This promotes working on goals. If I play a computer game, eventually I'll get bored or start to feel guilty. If I work on this homework assignment, eventually I'll get involved and forget how awful it is. Or: If I finish a computer game, I'll just go, What now? If I finish a homework assignment, I will feel proud and satisfied.

Change your criteria to promote better ways of living.