Postcard Mail marketing?

Is there any little things I can do to get a better response off my mail pieces? I send out 3000 pieces per month I would like to get a little better response. Is it better to use color paper? is it better to put a self stamp on? Please let me know your little tricks or even if you don't know what do you look at when you receive a postcard what grabs you attention? PLEASE POSTCARDS ONLY I DON'T NEED LIST OR TARGET MARKETS I HAVE THAT.