I have a strict daily word count, and I'll sit at my computer until I meet it, but the word count comes much faster some days than others. About a year ago, I read a great book called 2K-10K: Writing Faster, Writing Better, Writing More of What You Love, which was really helpful in this area. One of the author's many points is that when you're stuck in your writing, you probably either 1) don't know enough about that particular part or 2)don't find it interesting... which means your reader won't, either. Now I've wizened up a little, so as soon as I'm stuck, I'll look to see what in the plot is creating the block. But I should add that I'm not anywhere near 10K as a daily word count!