You need to introduce yourself briefly. State your occupation so your end-user doesn't think you're a shady Internet troll. If you're part of a larger organization, place your title and that org's name in your signature.
You need to express urgency. Give the user a deadline (3-5 business days from today) to provide an initial response by. State clearly you have other candidates lined up for the domain and are actively working to sell it. Mention you're willing to offer the end-user a price "below market value"; it sounds cheesy, but my response rate is higher when I use this expression than when I don't.
Finally, don't expect your initial response rate to top 15%, even if your domain is highly relevant to the end-users in question. Don't be disappointed if you send out ten e-mails and receive zero responses; conversely, don't register/catch a domain with the intention of flipping it in the first place unless your potential end-user prospects are sufficiently promising and numerous.
Use my sample e-mail on page 14, or one of the myriad of other pitches in this thread:
http://www.namepros.com/marketing-techniques/1757-domain-sales-letter-examples.html
Good luck.