State the purpose of the proposal. Don't make your readers guess what you're proposing; be clear about what you want the outcome to be from the start.
Pick your tone. If you are writing a business proposal, stick to formal language and back up everything you say with facts. You might use an emotional appeal if you are writing a charity-funding proposal. Try to establish that you are of good character and that your proposal has ethos.
Provide background information. Put yourself in your readers' place and ask what you need to know to understand the proposal. Don't assume your readers will agree with you, or your suggestions, so you have to convince them.
Use easy-to-understand English. Remember that nobody wants to have to go and look up a word in a dictionary to understand what you are trying to say. Clarify any terms your reader may not know.
Utilize a variety of sentence lengths in a three-two-one formula: three short sentences for every two medium-length ones and one long one. Keep your paragraphs short and remember that if you change ideas you have to change paragraphs.
Edit your proposal, then leave it for some time -- at least a day -- so that you can return to it from a critical perspective. Check your work for typos, grammar errors and consistent tone.
Ask a friend with good editing skills to review your proposal. It is easy to overlook where a proposal may be weak in one spot. Having an editor review it will make your good proposal better.