Successful proposals share a number of features. First, they should approach a theoretically interesting question. Purely descriptive research projects rarely are funded. The background and goals should be clearly and concisely presented. The materials and methods need to be well documented.
Many proposals fail because of loose ends.
The proposals should be carefully prepared. Although misspelling, sloppy citations and the like are not a reason for declination, they certainly do not create a favorable impression. It is always important to have a number of colleagues and professors critically appraise your proposal prior to submission. The more questions you deal with in advance, the fewer that can be pointed to in a review. Many researchers apply to a number of agencies for support of research.