My recent response to an article on another site discussing job search tips for Product Managers (thought it worth the re-post here):

As I have experienced with my clients regarding hiring product managers:

- They require VERY specific industry and product/technical knowledge and expertise
- MUST be good cross-functional and client facing communicators.
- Typically have well balanced technical and business capabilities
- On the resume, clearly defined examples of measurable successes.
- On the phone and in person, an ability to convey a strong passion for the work and industry, and able to communicate their methodologies and strategies in detail.

In short, to win at getting your PM job, position yourself as an expert in the vertical or technology, and articulate your successes well.

I'd say a cover letter is not a huge requirement. In fact, I hardly every see them any more and my client's don't ask for them. Most of what they will be looking for is on a resume. A cover letter, especially if not done right, can just be noise getting in the way of the real message. If you really would like to demonstrate your writing and communication skills, be prepared with a portfolio of sample white papers, technical marketing documents, lists of panel and industry discussions participated in, and links to any relevant postings or articles either written by you or about you.

Follow all that up with a solid networking strategy and you will have the foundation for a successful job search!