I'm going to tell you some methods to get more opt-ins to your e-mail lists, you may already know some or all of these, but I'm sure it will help out some list builders here!
You can make free giveaways or contests and advertise it in your signature (or in a thread in some cases).
2.) Free Bonus
You can ask other marketers to redirect their buyers to your page after they purchase something so you offer something for free, people will opt-in to download the "bonus". It will provide high quality (buyer) opt-ins.
3.) "Ask the Expert"
You can create a form where people can ask questions related to your niche, they will receive answers to their e-mails. Every question will be an opt-in.
4.) Solo Ads
This is one of the most effective ways to build your list fast, but it needs budget. You can reduce the advertising costs by offering a OTO to your subscribers.
You can exchange clicks with other list builders.
You can create videos related to your niche, offer freebies and tell them to visit your page.
That's all for now, I hope it helped! If you know other good methods (free/paid) don't forget to share below!