A few informative Guides for Writing Grant Proposals:
A free short tutorial on grant writing from the Foundation Center - http://foundationcenter.org/getstarted/tutorials/shortcourse/index.html
Detailed information on writing grant proposals from Non-profit Guides, who focus on grant-writing tools for non-profit organizations: http://www.npguides.org/
Another helpful site for writing successful grant proposals: http://www.mcf.org/nonprofits/successful-grant-proposal
Do you know of any other useful guides or have any tips on writing successful grant proposals?