I realize that there are two quite vocal and opposing camps on Warrior Forum in regards to the list building issue. The issue of whether or not to build lists is not the purpose of this thread, as there are plenty of other threads where that has been and is being debated. Let's just assume for the sake of mutual growth and education that anybody coming to an All-In-One thread about list building has accepted that there is value in having a list.
I'll start. The #1 lesson that I've learned in regards to list building is quality over quantity. Structure your opt-in offers to give you high quality leads, not just a body looking for a freebie. In my business, I have discovered that broader, generic offers have a significantly higher opt-in rate, but my much more niche-specific offers create a much higher quality opt-in in regards to their conversion into paying customers.
This is not my specific niche, but here is a parallel example. Let's say you have a copywriting course that you sell, and you're building an email list of copywriters. You're going to have a better quality list, and higher sales conversions later on, if you qualify the leads coming in. If your copywriting course, and ALL of your personal experience and expertise, comes from doing long-form direct mail sales letters, then the "widget" you offer to collect opt-ins should be specific to this.
If it's a generic "enter your email address for my free copywriting ebook", you're going to have a less selective list than if you offer "enter your email address for my free report revealing six steps to writing more effective direct mail sales letters". The latter will produce a lower opt-in rate, and fewer total opt-ins, but the list will be far more qualified for what you offer.
Brevity is not my strong suit, but I'll sum this tip up like this: Be specific in whom you target.
In line with the quality over quantity issue is the matter of how much information to ask for. In general, the more information you ask for from a visitor, the lower your opt-in rate will be. For my tax practice, I have some lead generation sites where I ask for everything including their mailing address, phone number, etc. Then I have other lead generation sites where I ask just for email address -- not even their name. Each landing page has a different objective. Make sure that the amount of information you ask for from a lead is in line with the objective for that particular list building campaign.
Here's to hoping this thread becomes a good list building resource for all Warriors!