One of my biggest challenges in writing is sticking to the topic. Depending on the length of the writing, sometimes you run out of things to talk about. As in one of my post before (writing about something interesting) plays a big part in this, if theres not anything interesting enough to write about you run out of things to say. I think the best way to stick to the topic is brainstorming; it can be creating bubble charts stemming points from major topic and creating other subtopics, a layout in essay form, or it can be just writing your thoughts down and putting it into your writing. Always asking myself how does all my ideas pertain to the topic also help.