When I have low motivation days I either:
1) Do really mini tasks that are easy to do and finish. That way my energy gets juiced up little by little with every mini accomplishment.
2) Chill. I don't know why but it seems to me like our minds, bodies and emotions need time to 'defrag' themselves and the whole system slows down when that's going on. Ever spent a day doing a whole lot of nothing and feeling totally refreshed the day after, or is it just me?
