Divide it into segments and keep it brief. Both make it more manageable from both ends. First paragraph: state what position you are applying for and that your resume is enclosed/attached. X looks like a great company for the following reasons and you believe your qualifications are a great match for the needs of X.
2nd paragraph: why you are a good match. This is the toughest. Don't wax poetic here. Keep it brief and concrete. A real example or two will suffice. "Organizing the HGTV RV Exposition for the past 5 years has showcased my talents for xyz as well as the RVs." You really only need 3-5 sentences max. It's a cover letter, not a tome.
3rd paragraph: wrap it up. "Should you have any questions regarding my qualifications, please do not hesitate to contact me. Thank you for this opportunity and I look forward to hearing from you soon."
DD -- 5YO
DS -- 3YO
Thanks all. Yeah, it's really that 2nd paragraph that drives me nuts (I use the same format). I always have trouble summarizing my experience and making it applicable, and all the examples online always sound so much better than what I end up with. Ah well, back to the drawing board.
I don't even know why these things are necessary. I do interviews for my company too and I usually just skip the cover letter and go straight to the resume.