I love the outdoors, I always bike, hike or kayak--if I am near the water. Then while I am doing this I think and think about the topic and the thoughts usually start flowing. Also, sometimes I will read something that I enjoy reading, like Southern nature writing (Usually Janisse Ray), then while I am reading this, the thoughts start pouring forth. I listen to my iPod for a while. Also, I talk with friends, I give them my points and ask opinions and sometimes get critiques of the subject matter, that way I will know some common ground that needs to be covered so that there are no gaps in my argument.
If all of this fails remember one thing: You are your own worst critic so, thinking about things too hard at first can lead to you tagging all ideas as "crap," so if you think about things too hard in the very beginning you are assuredly going to be devoid of thoughts, which leads to further discouragement. So just scratch it off layer by layer, then you will get to the good stuff.