"Sample" of one I used recently:
Subject: XXXXXXXX.XXX - domain name inquiry.
To:
[email protected]
Good afternoon!
I recently registered the domain name, "XXXXXXX.XXX" when it became suddenly and unexpectedly available to register. My long-term plans are to develop the domain/site, and I am now seeking input regarding development options.
Recently, I came upon your site:
http://www.xxxxxxxx.xxx during some of my research toward this end, and I was wondering if you'd be interested in acquiring this ".XXX" domain name to complement your site. Would this be of something of interest for you, if I had a fair and reasonable Asking price?
Though I treasure the domain name, my time commitments on other projects will likely force me to put these development plans on hold for the forseeable future. If you can utilize the domain to further enhance traffic and exposure to your site, I'd be willing to discuss the option with you. At your convenience, of course.
Thank you kindly for your consideration.
-----
Naturally, you have to modify your approach somewhat depending on the potential suitor, circumstances. The exact letter above has worked for me, and has not worked for me. Results will vary, persistency is key.
Hope that helps a bit.