One main thing I want you to do is not sound like a salesperson trying to sell or market something in your subject line. I prefer a toned down subject line. Similar to one you might use for a co-worker - it will be a description of the contents of the email, not something that is real persuasive.
My logic is that it is tough to create a subject line that motivates them to purchase, so create one that minimizes the instant delete.
Here is a webinar recording on email prospecting that may help: