Be concise. A good grant proposal should expunge all garrulous, flatulent and bombastic wordage and replace it with clear, succinct points. If it is concise rather than wordy, your proposal will convey its message and its purpose a whole lot better. It will also be less tiring for people to read. This de-cluttering process also includes removing jargon. Give the finished proposal to a friend who is not an expert in the field and ask him if he understands it.
Check your grant proposal for spelling, grammar and syntax. A grant proposal containing spelling and grammatical errors will appear to the person reading it to have been written by someone who is lazy. A basic word processor spelling and grammar checker will not catch all of the mistakes. For example, these checkers may not detect issues such as "their" versus "there" or recognize the situation in which a word is spelled correctly but is not the word intended (e.g., "four"/"for" or "chart"/"cart").
Factual errors can be divided into two areas -- contextual and budgetary. The contextual area has to do with facts relating to the problem, its context and the solution you wish to address with your grant money. The budgetary area concerns the amount of money you would need in order to address the subject. Errors in both areas seriously undermine the credibility of the applicant. Therefore it is crucial to double check all the facts and triple check the math.
The objectives of your grant must be quantifiable. The grant-issuing organization must be able to clearly understand what your objectives are. Filling the grant proposal with ambiguous notions and vague objectives will result in a rejection. Organizations want to know exactly what their money is going to be used for and why, as well as how the results can be measured. Be specific at all times.
Grant-issuing organizations produce grant proposal submission guidelines. As with most magazines and other publishers, when a manuscript fails to follow protocol it is automatically rejected. There are no special cases, no special circumstances and no excuses for not following the clearly laid out guidelines. So, read the guidelines, understand them and ensure the proposal meets every criterion specified.