Being an interruptive approach, email marketing necessitates the creation of messaging with value. Spray and pray is a thing of the past while targeted campaigns which are drip fed consistently over time is an essential technique. Here's some tips on email messaging...
Subject Line Appropriateness
Set the content expectation
Giving a wacky or urgent subject line will definitely get a large open rate (high percentage of emails being opened). It will also result in a terrible click through rate (low percentage of people acting on the email). It will also results in a large group of people not opening email from the sender, ever again. So, set the tone, stay on topic and use a subject line that creates a perception or expectation that is going to be continued throughout the body of the email. Be consistent.
Make it easy to read
Do not bother with odd type-faces, over-use of punctuation or other detractors. Make sure to space between paragraphs and make the message easy to skim and scan, since that's what most people tend to do regularly. Easy reading is appreciated, so remember to use bullet lists and short sentences - save the prose for the Shakespeare Festival.
Infrequent messages get less response
Stay in front of your audience by regularly reaching out to them. Even if it's a quick paragraph with a link to some valuable information, at least you're staying front of mind and building the reputation as giving value to the reader. Less than 1 contact per month will result in lower open rates and lower conversions.