I would say keeping plot details in order. With fantasy, there is a lot of world-building. Even if not all the details end up on the page, there are details that I must have organized in order ensure there are no plot holes. I still run into plot issues during editing and have to go through the whole book to figure out how to fix them. Thankfully, I had a wonderful editor for the Arlin Trilogy who did a lot of the locating of plot issues, so between the two of us it worked. But it is a challenge to keep all the details in order.
I experienced writer's block in college and quickly realized I was not going to get through all the papers I had to write if I kept having that issue (I have a degree in psychology, so lots of writing!). So, I taught myself to simply start writing. Now, when I start a project, no matter how bad it is, I just get words on the page. Once I get words on the page, thoughts flow faster, and details come to me, and I can go back and fix the beginning in editing. If I think too much when I first start a project, I will get stuck. So, no matter how good or bad the first chapter is, I just dive into the first draft knowing that I will go back and fix it once I get the story flowing. I have literally had moments when I was typing and thinking, "this makes no sense and will not work" but I didn't let myself stop typing until something came to mind that would work. Many times, this "something" is just a flash of an image or feeling that I want to convey. Once that happens, I either write about that idea/feeling, or I write down that thought and take a break before coming back and continuing the process. Usually, once I get that image figured out, the rest is much easier.
