You need to be sure that your cover letter follows standard letter format. With the proliferation of templates (Microsoft Word has great ones) this should be the easiest step. Find a business letter template and follow it.
Next, be sure your lead introduces you and is a direct statement of purpose. Tell them what job you are applying for and tell them you want it. Be very direct and write clearly and concisely. Don’t make the reader guess why you’re writing or how your skills match the position.
Now you need to make your pitch. What skills do you have that will benefit this company or organization? List three or four bullet pointed highlights. Don't just rewrite what is already on your resume. Try to tailor your experience to the companies needs in order to make you cover letter stand out.
Finally, thank the reader for his time and state that you look forward to meeting them to discuss the position in person. Be forceful without being pushy.