Take a piece of paper + write yourself a love note. Jot down all the things you LOVE about yourself on this note, don't let a negative thought move from your mind down to the paper. Tuck this note away and on a day when you aren't feeling your best, take it out and remember all the great things about you.
If you need to, read it twice, read it three times, read until you feel better.
And if you see someone having a rough day, suggest writing a love note. :)