Be concise, but complete. 

Yes, those sound like contradictory statements but they are not. Reread the first draft of your proposal with that statement in mind. Some people are natural writers and can write five pages at the drop of a hat about their sleeping cat Fred, who is a wonderful black and white mix they got from the pound two years ago last October. You get the point. Those folks may need to focus on the concise part. Other writers assume that the readers know a lot about the topic and leave out too much material. Those folks may have to work on making sure the proposal has enough information for reviewers not familiar with the topic.

Proofread before submission.

Be sure to have at least one other person read the grant proposal before you submit it. Yes, this means you will have to finish the proposal sooner than the night before it is due. But think of it this way: you now have somewhere to direct the energy of a bossy, know-it-all neighbor/friend. Ask him or her to read it over and give you suggestions. He or she should not only look for typographical errors and erratic syntax but also consider the overall flow and logic of the proposal. If you cannot submit an error-free proposal, how can the review committee be confident that you can do the work correctly?

Good luck!