OMG!!! You are preaching to the choir!!! I run into that wall all the time!!! I know what I want to say and how I want to say it, but then as soon as I sit down to start typing it out, I am at a loss as to how to begin. I don't have a tried and true method that works every time, but what I found helps are several different things, one is step away from it and collect your thoughts, you know what you want to say,it is just how to draft the beginning. Give yourself a few minutes to think just about the beginning and how you want to introduce it, then jot it down, you can always go back and edit it (and you will almost guaranteed), once you get on a roll. Another method I sometimes use if I am comfortable with the real "meat" of the document is go ahead and begin drafting that and skipping the intro for the time being. Usually once you begin writing, then a good intro will lend itself to you and you can go back and plug it in. Another way is to just write out a simple purpose sentence to get you started, what is the ultimate goal of this document, why am I writing it and what do I intend to achieve with it. Most importantly is to relax, if it isn't coming to you, let yourself become distracted from it for a little bit, you will find that while you were working on other things you were still thinking about it and forming it in your mind. The worst thing you can do is try to force it because if the words aren't going to come, they aren't going to come and it will only be painful and annoying and you will end up trashing it all and beginning over again, (yeah, I'm talking from excruciating experience). Hope this helps, but know you are not alone, there are many of us out here that deal with the same thing.