all about the brush.. if you are beginner, i think you should check out shisheido. They have this hard brush that can be used as a pen. it gives you a frim grip so easy to "draw" the lines and it also has an ink control so it wont be messy all over your eyelid
if you are an expert, you can get a soft brush. I've been using an escada liquid liner. Thin brush doesnt get apart easily