I was fortunate to have my grant proofread by someone who served on the study sections reviewing these grants. His advice was invaluable, and really changed the way I approached these grants. Your grant in itself needs to be reasonably well written (especially the specific aims page), but the bulk of your score comes from the training environment. The grant is not really funding your project. It is funding your career development. Your PI should have a good track record of training trainees. Your PI needs to outline exactly how he plans on mentoring you (e.g. plans to meet with you weekly). You need to outline the members of your thesis committee and each one can help your project. If your PI is not high powered, it will help to put someone high powered on your committee. It is even better if that person writes you a letter of support. You need to outline all the resources available at your institution (a mouse genetics core? Three atomic force microscopes? etc). Really focus on how your PI, your committee, and your institution are going to help you do great things. And if all possible, find someone who reviews these grants and get their honest feedback.