This is a good question. I'm by no means an expert, but I can tell you what is and isn't working for me.
A few things: My list is small, but has been scrubbed of freebie seekers, and the audience is super engaged. In my mind, It isn't about having a huge number of subs. It's about having engaged subscribers who genuinely like your work. Even if that list is much smaller, it's much more valuable (to me).
I increased my subscribers and engagements AFTER:
I spent the time to completely retool my mailing list after reading Newsletter Ninja. Namely, the advice to send content that is 'valuable'. This means something different depending on genre and who you are as an author. But I feel like if you use the word "genuine" and "entertaining" interchangeably for "valuable," you're on the right track.
I read "Strangers to Superfans". It helped me fine tune. Ultimately you have to know who your ideal reader is and what they're interested in. As soon as you know that and can give it to them? You've got the key to success. (Aligning your genre with your personality helps a lot...)
So, I revamped the emails I sent with all this in mind ^^. Then, I added the sign up link to the front and back matter of all my books (As a clickable link but also spelling out the actual form address, because people don't like to use the Kindle browser and will type that address into their computer later), which has increased sign ups. (I currently don't have a reader magnet.)
Other than that, I've stopped following the 'normal' advice.
I've stopped doing newsletter swaps. Now, I only recommend books I have personally read and enjoyed. I don't get the benefit of swaps, BUT this has super duper increased trust and engagement. Probably because they don't think I'm just trying to sell them something. I'm like a friend who knows what they like and shares good finds.
I also don't do much Bookfunnel or StoryOrigin, mostly because it's really hard to find good genre matches outside of romance and UF. And, I was the admin of a few promotions last year, and you'd be amazed how many people NEVER share the promotion to their list or even post it just once on social media. I may try again once I have a reader magnet, but I will use it selectively and carefully manage the resulting list.
So what do I do?
Ultimately, I rely on my organic list. Your organic list is always your best list, although it is frustrating that it takes so long to build! I do what I can to speed that up by putting the signup link where people can see it, optimizing it, and emphasizing the value they will get out of joining me.
One tip though: If you ask people to interact--like hit reply to answer a question or enter a raffle--that somehow increases deliverability and your "reputation" as a sender, which I *think* translates into fewer emails in the spam filter.
I also focus on moving the people who found me on social media after reading my books onto my organic list--in time-- with the clever use of carrots. I do giveaways only for fans who are only on my list. And I post teaser graphics for all my giveaways, along with a sign up link. They go to a separate dedicated list. If people engage over time, they get moved to my main organic list. The giveaway drawing is on YouTube. (Which also brings in new people.)
The giveaways are fun and don't cost too much, because they tie into my genre and my books. Because I know who my ideal readers are, and what they like, so I have a pretty good sense of what giveaway items will get them excited. (Which is why the Strangers to Superfans formula works).
It's the slow steady build method, but it's worked out waaaaay better than the quick build methods. I've done a lot of things that didn't work out great, that I will never do again.
-Like paid $60 toward a giveaway of merch for fans of a big trad pubbed author, only to get 1500 sign ups whose emails also went to 11 OTHER authors who also paid to be part of the giveaway. Pretty much all those people unsubbed, because they were inundated with author emails.
-I've done rafflecopters combined with blog tours to net me followers that never really engaged. They just wanted the prize.
-I've put a first in series free on Bookfunnel and story origin. I kept about 20 percent of those people ultimately. The rest just wanted free stuff.
I treat my organic people like the treasures they are. I never give away free books, but I am generous with time, recommendations, responding to messages, and in deleted scenes, etc.
I launch at full price, and I don't discount my books very often. When I do have a sale, people don't hesitate to grab it. Ultimately, your best list builder is having books that really deliver what readers in your genre want in a way that is uniquely you. AND the best way to keep them is to send them an email they LOOK FORWARD TO GETTING.