A Subject Line Checklist
Assuming an email gets delivered AND the contact sees it, the first thing impacting the open rate is the subject line. Here's a quick checklist of things to consider as you build effective subject lines.
Length of Subject Line
Keep the subject line short – 7 words and 41 characters are optimal.
Use emojis to help your subject line stand out.
Make the subject line unique to the recipient.
Take the time to A/B test your subject lines.
Find the balance between trendy and personalized.
Word Choice in Subject Line
Establish Urgency 
Limited number available
Don’t miss this
Create Curiosity 
Ask a curiosity-inducing question
Start a sequence of events, but don’t finish (e.g. an unfinished story)
Imply that you have information they don’t
Imply that they used to know something that they’ve since forgotten
 Loewenstein, G. (1994). The psychology of curiosity: A review and reinterpretation. Psychological bulletin, 116(1), 75.