I use this one with decent results. Although the first reply is usualy:"How much?" so, if you know what you want for the domain, it is good practice to list the price in your email.
Dear [bla],
Our company spends quite a bit of time, money, and resources acquiring generic domain names. [DomainName.com] has already generated much interest from other parties.
We have plans to develop this domain name for our own use, but we are considering offers. Within 10 days, we will either accept the highest offer or continue with our plans to develop this domain. Thank you for any interest you may have.
Please let us know if you would like to make an offer before we finalize our plans.
Sincerely,
Bla Bla Bla
This is a one time mailing. We will not contact you again unless we receive a reply to this correspondence. Thanks again.