There are many resources available for developing your proposal writing skills.  

Here are some of the best:

A Practical Guide for Writing Proposals
Alice Reid

Foundation Center Learning Lab -- Virtual Classroom
Foundation Center

Grant Proposal.com

Grant Proposal Writing Guides
United Nations

Grantwriting Tools for Non-profit Organizations
Non-profit Guides

Guide for Writing a Funding Proposal
S. Joseph Levine, Michigan State University

Guidebook for New Principal Investigators
Canadian Institutes of Health Research

Proposal Checklist -- Social Sciences
Deborah Kluge

Proposal Pointers, Tips and Pitfalls
Deborah Kluge

Proposal Preparation
Small Business High Technology Institute

Proposal Writing Short Course
Foundation Center

Teitel, Martin.  The Ultimate Insider's Guide to Winning Foundation Grants: A Foundation Ceo Reveals the Secrets You Need to Know. Medfield, MA: Emerson & Church Publishers, 2012.

Geever, Jane C. The Foundation Center’s Guide to Proposal Writing, 5th edition. New York: The Foundation Center, 2007.

Browning, Beverly A. Grant Writing for Dummies, 2nd edition. New York: Wiley Publishing, 2005.