I guess for me since I'm writing about my fantasies that it's always stimulating no matter how often I go over it in my head or read it written down. It's the reason why I've always had intentions of writing but could never get to the actual writing stage. What should happen is:
Picture the scene in my head, add descriptive words, write it down

Which became:
Picture scene in my head... fap, fap, fap

I've actually found that writing in a public place helps me focus a little, since I know that I can't do anything to relieve my tensions (without getting arrested anyway) it gives me a little more willpower.