There are a variety of organizations and associations that cater to helping the development of proposal writers.  Through these groups you can meet other people in your field, learn new techniques and advance your career.

Check them out and make sure to let them know who sent you. 

Association of Proposal Management Professionals

Association of Small Business Development Centers

National Contract Management Association

American Bar Association – Public Contract Law Section

TECHWR-L
Web-site supporting the technical communication community

Project Management Institute

Society for Technical Communication

Society for Marketing Professional Services