if you have a list or maybe it's a huge list and you have few and little open rates, the problem definitely on you. if there is anyone opt in to your list because maybe they have interested in your lead magnet. chances are that they on other marketers lists also I'm I correct, think of it logically. if they on other marketers lists how you can separate yourself from this competition. look at what typical marketers do they always pitch since anyone opt in their emails lists. and because of that their open rates always decrease until it disappear, if anybody opt in to your list don't pitch them on the first 5 or 6 emails, provide them value, make them know who you are as a person, with that you will build a good relationship with your list.
Maybe you heard this argument "MONEY ON THE LIST" but this isn't true I'm saying "MONEY ON THE RELATIONSHIP WITH YOUR LIST", you need your prospects to be responsive to your emails and you can do that with providing them value, free stuff. I don't say that you don't try to sell them.
as long as you have good relation with your list. you will make more open rates and more sales.
This is it, this is just is, this is the most important factor of increasing your open rates.
I wouldn't talk about good title or short email. because if people know you will they will open your emails whatever title you have.
I hope this question was helpful to you. let me know what do you think by replies.