To write a good proposal takes a long time. Start early. Begin thinking about your topic well in advance and make it a habit to collect references while you work on other tasks. Write a first draft at least three months in advance, revise it, show it to colleagues. Let it gather a little dust, collect colleagues' comments, revise it again. If you have a chance, share it with a seminar or similar group; the debate should help you anticipate what reviewers will eventually think. Revise the text again for substance. Go over the language, style, and form. Resharpen your opening paragraph or first page so that it drives home exactly what you mean as effectively as possible.