First of all, don't rule out the possibility that it might not be just the subject line that's tripping you up. There are other factors - timing, list fatigue, your mail is getting caught in a spam filter, your list is crappy, and the biggest one of all - your content isn't compelling. That said, subject lines are really important. People like offers but in my experience if the subject line is too sales-y the open rates drop. Every audience is different so you will have to do some A/B testing on subject lines to figure out what works for your audience. There are loads of people writing about copy writing online right now that are experts in this stuff that you can learn from.