If you are pitching your idea or services to multiple parties, the one thing you do not want to do is send out a general form letter along with a standard brochure or stack of resumes. That sort of 'one size fits all' approach cannot substitute for a real proposal. The goal of a proposal is to persuade the client or boss to endorse your idea and let you do the job. To succeed at convincing them, you need to focus your message to a specific situation, gain their trust and show them that you know what you're talking about and can deliver what they need.
The introduction section is the shortest. Start out with a Cover Letter and a Title Page. Keep the Cover Letter brief: simply write a personal introduction to explain who you are and provide your contact information. The Title Page should be exactly what it sounds like: a title that introduces your proposal and provides a clear message about the ideas and/or services you are pitching. Some examples might be "Proposal to Create a New Executive Assistant Position", "Proposed Temporary Services to Benefit the Stuart Corporation", "Executive Search Services Proposed for Jameson Company", or "Suggested Candidates for the Vice President Position".