Whenever you are feeling down. It is best to step away from distractions(phone, computer,TV), and take some time to yourself. Give yourself time to reflect, and write. Listen to music you enjoy. Do other things you may enjoy. You can also try going for a walk, if it is nice outside.
The point is to clear your mind from the negative state it is in. Step away, and be in the moment. This gives you the space to bring your mind back to a more positive state. Doing this always helps me bring my mood back up, and refreshes my mind.
Psychology of Mindfulness 