I always like having some ice cream, or taking a long bath. If it's relatively nice out, taking a walk in the evening or going to the park with a friend is always nice. Even eating a tasty, somewhat healthy meal is good.

My favorite, however, is sitting in and watching a show from my youth, like Pokemon or something. You can find many TV shows on the surfthechannel.com, so you can pick whatever you want to watch.

Having a girls' night with your friends is a great pick me up. Maybe snuggling up and watching your favorite movie, whether little kids or a comedy, whatever makes you feel happy. If you're a bit frazzled or angry I really like video games, it helps me take out my aggression, but I understand if you're not into that kinda stuff. A bath or walk both help me relax a bit as well. Also, a good book is a great way to stop focusing on all the little negative things going in your life, gets you focused on the story and characters and less on your own problems.
Hope these were helpful!

Buy/pick yourself some nice flowers, listen to your favourite music, eat your favourite foods (I agree with Emma, something delicious and healthy is great because you won't feel any guilt!), beautify yourself - hair treatment, face mask, whatever makes you feel good, call/visit your best friend, a walk in the sunshine, play with yours/somebody else's pets, read your favourite magazine, re-read an old favourite book, light some scented candles and chill, ask a friend to tell you what it is they love about you - a total boost to the ego - perhaps even write some of these things down so you can look at it when you're feeling down....do whatever it is that makes YOU happy and forget pleasing other people for a while =)
I made a pretty poster for my bedroom wall full of pictures of the things I love, inspiring messages and a list of "all the beautiful things in this world" ie: my friends, random drives on sunny Sundays, lying under the stars, my boyfriend's cuddles, the beach, mascara, my cat, snuggling up with a hot chocolate on a cold night, etc - looking at it NEVER fails to lift my spirits!

And what not to do? Steer clear of people who don't accept you for being you and make sure to surround yourself with positive people whose company you adore - happiness can be contagious!

for me my friends or bf always help if im in a mood theyll tell me to get my shit and lets go. they really say that haha. and take me out to have fun. but when im alone i just play happy loud music and play with my almost year old puppy

When its chucking it down with rain in the evening, run yourself a hot-as-you-can-take-it bubble bath, with candles (Make sure they arent next to anything that can catch alight!!) a nice mug of hot chocolate (Or your favourite drink) and a favourite book. Open the bathroom window a little so that you can feel the gentle breeze of the wind, and hear the rain pounding down. Turn out the lights, sit back in the hot bubbles and relax!! Let go of every tension in your body and let the water surround you.

When you want to get out - make sure you have a warm snuggly bath robe waiting for you to get into.

Enjoy!