Study the Sponsor
This may appear obvious however it is one of the most common reasons countless applications do not even make it to the review process. Ensure you are reading the entire Request for Proposals (RFP) as well as learning a bit about the sponsor organization. Your project needs to not only align with the RFP in question but with the core values and mission of the sponsor organization. This is doubly true when looking for public funding, as government agencies are required to only fund the projects of which strictly fit into what their mission is.
Example: National Cancer Institute. Lets use an NCI grant for this example. From the main website you can navigate to the linked page and read in detail about exactly what type of work the NCI funds. Make sure to look for mention of current funded organizations and projects as well as sections directly discussing what they look for when releasing grants. Not every site will have all of these specifics, however the vast majority will give you some degree of insight into if your project will fit.