Work on your posture. Walking upright with your back straight helps you feel more confident and positive.
Find out what you really love or are good at. That helps bring positivity about yourself.
Remove any source of negativity. By this, I mean unappreciative people. Anyone who can bring you down. You don't need them and you don't deserve them.
Read self help books. And not the ones that preach, but those that tell anecdotes. They are more interesting and you'll remember them for much longer.
Implemented all these when I was in a rut and they helped. Hope they do the same to you :)