厚唇 is a nice thing la. nowaday,this kind of girls look more sexy and beautiful,you can say it is a trendy look. You can see many fashion manganizes from all over the world that they teach you how to make your lip look more thick!!
But if you really want to make your lip look thinner,you should use DARK colour,remember DON'T use light colour. And please don't try to use any lip gross or shinny lipstick,because it will highlight your lip and people will focus on your lip.
I think cheaper and good comment 好用既唇膏: Maybelline or Red earth
reasonable price and good comment : RMK