I save them in my wallet and probably when I have many I will do an investment. That would be a good idea. 
This is what i try to do but in the end it all just goes back to the websites that gave it to me like Primedice or Rollin but i can't really change that too much because it was free money in the first place after all.
I try to spend it when it's high though to feel good about spending less.