At one point, I saw a meme that resonated very strongly with me. The gist of the meme (because I doubt my Google-fu is strong enough to find it again) was that writer's were the only ones who used not feeling like working as their excuse for not doing so. Now, granted, this is a vast over simplification of writer's block. Still, it was something that struck me as a powerful argument. I have had people approach me and declare that the couldn't be a writer because they couldn't finish something.
I always responded with a question. That simple questions was one word: why? That was about when the conversation stopped because I challenged their position of inability. When I hit the point where I am blocked, I ask myself that same question. I feel like I can't write. I feel like nothing I write is good enough. It is generally a pretty horrible feeling, regardless of weather or not you are attempting to make writing your profession.
Usually my argument with myself is pretty simple. I have my laundry list of negative self image problems arrayed on one side and the evidence that I can write a novel (or what ever else I happen to be struggling with) on the other side. Somewhere along the way, it becomes clear that my feeling blocked is rooted in some type of fear, sense of being overwhelmed, or depression. Someone else's triggers for feeling blocked are going to be different from mine, but I am sure that as you dig down through those messy feelings, you are going to find a pattern to them. After I have isolated what the cause of my block is, I recognize that I can resolved it by addressing the problem that lies at the heart of it all.
In my case, the majority of my writer's block cases manifest as perfectionism. I find myself expecting my first drafts to come out with all the polish and sophistication of a finished product. I find myself expecting that my characters are going to be fully developed the moment I present them on the page. We won't talk about how harsh I get with myself over the way things are constructed on the page. I will only say that the world should be thankful that I didn't pour my efforts into perfecting grammar. I am a little neurotic when I get anxious or stressed out.
How do I resolve the problems that arise from my perfectionism? The biggest thing is giving myself permission to write badly. I also make a point of giving myself low stress writing time. This means I am working on background material or personal journal writing. Things that I will not be showing for public consumption or otherwise has a great deal of weight attached to it are what I work on when the block is particularly bad. Some times, we have to take a break from the 'important' projects to rest. Now, depending on what you are working on, that break may be a five minute journal entry between projects or it may be a short story. Either way, the break gives a bit of breathing room and an opportunity to lower ones stress levels, which gives the opportunity to return to the main project with a fresh perspective.