If you have not come across it yet, check out this post at Ask a Manager to see what a good cover letter looks like. It is a post with great information and don't forgo the comments for added perspective. Above all, be yourself and sell yourself - you know you have much to offer and this one page document can help you shine. Believe me, as a person who has seen hundreds (ok, thousands) of resumes with and without cover letters over the years, a little differentiation will go a long way. Shine away!