Sethi said the best format to follow is introduce yourself, reach a commonality, and then ask a question. Whether you do this in a single line or short paragraph depends on the recipient.
You'll want to have them take a look at your message and be able to give an adequate response, even if it takes them 30 seconds on their smartphone. When Levy emails a high-demand person like a celebrity, he keeps his email down to one sentence that cuts out any trace of filler.
If he emails an executive, who makes decisions based on available information, he'll limit his message to three to five sentences and include some links they can click if they'd like to learn more about him and his organization.
Sethi and Ferriss also said that the most important thing to remember is to be respectful of the power dynamic between you and your recipient. Don't resent them for their saturated schedule, and be grateful if their reply is even a few words long.