2. The "How do you ..." subject line.

Examples are:

How to Get More Visitors to Your Website in 5 Simple Steps?

How to receive a terrific paycheck within a few months?

How to easily earn hundreds of euros a year with your laptop?

The "How to" is an evergreen opening sentence, not just in e-mail marketing.

Check out the magazines, look at the covers. You will see several magazines with those types of titles on the cover page. They know it sells more magazines.

Please note that such a subject line always tells about the end result, not about the effort.

How to is implying that it can be achieved
("5 simple steps", "easy", "within a few months").