There are two resources for the grant developer and writer: general guides published by nonpartisan organizations like the Carnegie Corporation of New York, and specific guides published by agencies like the National Science Foundation or U.S. Department of Education. Below are links to some of the most widely-used guides of both kinds. In these guides you should discover advice, examples, and tips.

General Grants Writing Guides
The Catalog of Federal Domestic Assistance, which oversees all federal grant making, publishes a detailed guide to Proposal Writing.
Grant.Proposal.Com provides guidelines to writing full proposals and letters of inquiry, general tips in grant seeking, advice from major funders, and more.
Guide for Writing a Funding Proposal, written by Professor S. Joseph Levine of Michigan State University for LearnerAssociates.net, covers the basic elements of a proposal writing, and provides links to other writing guides.
The Nonprofit.Guides includes Grant Writing Tools in their series of publications offered to help applicants through all stages of the grant writing process.
Resources To Help Write a Proposal is the Carnegie Corporation's portal to multiple guides helpful to grant writers at all levels and stages.

Agency-Specific Grants Writing Guides
National Endowment for the Arts
Lessons Learned: A Planning Toolkit will take an applicant through the entire life cycle of a grant, from the very first planning stages to finalizing a detailed budget. Designing outcomes-based evaluation is often a challenge for arts grants, so the NEA also provides a special Outcomes Evaluation Toolkit for its applicants.

National Institute of Health (NIH)
All About Grants tutorials help biomedical investigators, especially new ones, plan, write, and apply for the basic NIH research project grant.
Common Mistakes in NIH Applications could save an applicant needless rejection and revision....at the NIH or elsewhere.
Grant Writing Tip Sheets are useful when applying for grants through the variety of NIH sub-agencies.
Quick Guide for Grant Applications can jumpstart your approach to proposal development.

National Science Foundation (NSF)
Grant Proposal Guide is a comprehensive guide for all applicants submitting to the NSF.
Guide for Proposal Writing gathers the best advice given to applicants by the NSF staff, from program directors to panel reviewers.

Resource Management Systems (IT)
IT Proposal Preparation Primer walks an applicant through the steps critical to an effective grant package.

Social Science Research Council
The Art of Writing Proposals provides “Candid Suggestions for Applicants.”

U.S. Environmental Protection Agency
Grant-Writing Tutorial is a joint effort between Purdue University and the EPA to describe the nine basic components of a successful proposal package.

Fulbright Program
The Fulbright Program provides a number of "writing tips" for its international programs:
Guidelines for preparing the application:
http://www.cies.org/us_scholars/guidelines.htm. 
A PowerPoint presentation about awards and preparing a successful application: http://www.cies.org/campus_reps/download/FulbrightWorkshop1.ppt. 
See also the Tips for Applying: http://www.cies.org/us_scholars/tips.htm.