This might sound obvious, but one of the most common mistakes for beginning grant-writers is to attempt to sell a project on its merits instead of following the simple instructions requested of all proposals. The last grant took myself, and a friend, ten days to write and was 15 pages long. My first grant was won after four years learning the following lessons. Talk to those who got funded and ask for advice and ideally copies of their successful grants. Read the current guidelines for those foundations on what they will fund and when the grants are due.
Foundations often shift their focus, and timing can be very important.
Watch for timing sensitive opportunities. Collect sample successful grants to use as boilerplate models. Many foundations will send you, on request, proposals from past funded projects, or at least the addresses of past grant recipients, so you can ask them directly for copies of successful proposals. The more good proposals you read, the more you'll understand how clear writing and following guidelines leads to funding. Use the same terms in your proposal that the foundation used to describe what they want to fund.