Assuming that you've got a decent subject header, more on that another time, then there is a reasonable formula you can follow.
Provide evidence of similarity or social connection, eg a mutual problem or contact
Understand the recipients point of view, eg why its in their interest to respond
Why ere they selected, eg a bit of flattery if they're an expert
Be familiar with the subject/topic that the mail is about. No one whats to reply to a mail if the sender has shown no effort to research, you might even get an RTFM response