I wish I had kept track of how many rewrites and edits I have made to what is now a finished dissertation proposal. Well, actually, one rewrite and several edits of awkward sentences and muddled thought. It has been an interesting process and I wish I had done more editing of previous work. It really does make a difference to go through the text a few times and find the weak spots, particularly because you have time to get some distance between yourself and the text. That way, you can see what actually ended up on the paper and not just what you intended to put there.
What have I learned about writing a proposal?
Start by setting up the items that need to be covered and then start filling them in one by one, not necessarily in the right order. It really helps to have something on the page. Write at least a paragraph about what you want to do/explore/investigate. Then figure out something about WHY you are interested in this issue - what is missing in the research so far that makes this the next step.
Also, using secondary sources well is difficult. You have to define their place in your work and define what sources you are leaning on for insights OR for methods, what sources you are disagreeing with and what sources you are going to ignore after you have explained why you are going to ignore them.