Ask HN: What was your most effective email subject line?

We're sending emails with new beta builds every 2 weeks. And after the 10th email I lost creativity. I tried many combinations from AppSumo website, but have 0 ideas now. Could you please share email headlines that you have used? (for product launches or product updates) I'm waiting that day, when we will be able to hire a native speaking copywriter.... Thanks :) ^ Lisa