Check whether there are any formal requirements for the proposal. These may relate to: the wordage, the way the proposal is laid out and what must be included in it. There may also, in a grant proposal or business proposal, be a requirement to include costs. The most obvious mistake in writing a proposal is to disregard this vital information.
Ask the appropriate person for advice on writing the proposal. This will be a professor, or adviser in an academic situation. In business, ask your manager for any tips, advice or help. If you wish to write a book proposal, ask the advice of a writing group and research relevant articles in writing books or magazines.
Look at sample proposals. Your senior manager may provide you with draft and finished proposals, in a business situation. Study these thoroughly. Look for relevant books on writing proposals. You will find a choice, some of which will be more useful than others to your particular needs. A range of online sites offer guidance on writing specific types of proposals.