I've been there.

When I'm run down and demotivated, I plan exactly what I'm going to do the next day to turn my mentality around. For me, a sense of accomplishment and productivity is essential in getting my mind right again. First I make a list of what I'm going to do. What needs done? Does the house need cleaned? Does laundry need done? Do bills need paid or do you have an assignment to finish? I need to physically write a list. Then I'll set my alarm and wake up early (this is important, because sleeping in makes the lethargy worse) the next day, have a strong cup of coffee with breakfast, and work out. Then, like 8bit said, I often meditate after a shower. Tackle the day! Start with easy stuff to get your momentum going. Take the trash out, do the dishes, and as you go, cross them off your list. Keep going until your list is done, and reward yourself at the end of the day with something- a movie, a beer, some time with friends. You'll feel like you earned it instead of like you're using it to escape time and obligations.