First, don't be too hard on yourself. Most of the time this is where writer's block stems from. You become so concerned with being able to write eloquently it prevents you from starting. The time for criticizing yourself is when you do your editing. Just write. Worry about the finessing later. Often what I'll do is just start to summarize the plot of what I want to happen. Eventually, what will happen is the summarizing will start to turn into actual writing and, at some point, I'll be able to go back and fix the beginning of it.
What I've also done in the past to help me along was create achart of the five senses. The titles along the top of the columns would read "See", "Taste", "Touch", "Hear" and "Smell" and under each column I would list what my character might be experiencing for each. I would do one for each character, but I would focus the most on the spreadsheet for my POV character. I wouldn't use everything I wrote down, but it worked well to help me put them in their surroundings and, therefore, help me more effectively write the scene.
Something else I find helps me is I have more than one writing project to work on. When I find myself feeling stuck with one, I put it aside and work on something else. At times, writer's block can stem from boredom.
