Become very familiar with the requirements of the proposal. Before sitting down to write, familiarize yourself with every aspect of the proposal requirements: length, font-size, margins, questions to be answered, sections, and so forth. Failure to follow basic instructions might be reason to disqualify an otherwise adequate candidate -- especially if the contest is competitive.
Carry out adequate research. Spend time researching the institution -- who is there, how do they think, what do they expect -- and the material that will be used for the proposal. When including research in the proposal itself, be very selective: use only the most trusted and verified sources.
Begin by defining the program or how you fit into it. Depending on whether you are proposing a program or trying to get into one, you must demonstrate immediately why it is important and/or how you fit into it. The first paragraph is very important, since first impressions will tend to color later impressions. Start strong to avoid sowing doubt from the outset.
Explain how the program will work or expand on what you intend to study. If proposing a program, explain some details of how it will work, who it will include, and how much it will cost. If seeking to enter a program, explain what you intend to study, with whom, and why this institution is the best one for you to attend in order to complete your project.
Edit the document closely. If you write more than a page on your first draft, don't worry. Take the time to write everything out, then go back to edit it closely. Check for spelling, grammar, paragraph structure, wording, format, and overall coherence. Read it aloud to yourself or a friend; if there are any awkward passages, you're more likely to catch them when you hear them read aloud.