The good, modern salesperson is knowledgeable, conversational, persistent, and honest about the product. The good, modern salesperson does not speak the language of hype but talks to the prospect or repeat customer as though they were having a cup of coffee together. Your
email marketing and
email newsletters should model this friendlier, more relaxed type of interaction.
Avoid ALL CAPITAL LETTERS in the subject line as that is widely thought of as the Internet equivalent of shouting. You would never shout at a prospect so do not shout at your email list subscribers. Avoid exclamation marks!! When I see exclamation marks I think of Mr. Polyester Suit. It is worth noting that if it looks like hype to you, the spam filters probably agree. For example, the filters are likely to flag emails that have ALL CAPS or exclamation marks and send them to the junk folder.
A good litmus test for a subject line and for email copy writing in general is does it past the read test? If you read it out loud does it sound even vaguely how you would talk to the customer over coffee?