There are hundreds, or even thousands of websites and ezine articles about list building secrets and while some may be of value, most are usually junk or relate to specific products and may not be of any use to you. To get you started, here are some simple list building secrets - you can go on to more advanced techniques are you become more adept at the process.
The first secret is to forget about gigantic lists - people will tell you about how to create lists with thousands of names but unless these names are those who are serious potential customers, what good are they to you? However, a smaller list of say 500 names, the majority of whom are potential customers is something of much greater value - you will know for certain what your market is and can develop better strategies to sell to it.
To build up your list create a squeeze page that offers a free download related to your line of business to those who will leave their email addresses - people who do this are people who have an interest in your products. Once you have a small list, ensure that the content you send them is of the highest quality and is sent regularly (that is the only way to keep their interest) - you need to continually remind those on you list that you exist and the material you send them must provide proof of your seriousness and the quality of what you are offering them.
Once you have established a relationship with those on your list, you can start asking them for references - many will not respond but those who do will be the ones who see value in what you are offering and would like to give the benefit of that value to their friends - their recommendation will mean that half your selling is already done before you even send them your first mail. And do not stop with email addresses - discretely ask for more information to build up customer profiles: but ensure that you are not seen as being nosy as this could lead to your motives being questioned. Use surveys and opinion polls to get the information you want without appearing to be nosy.
You can pick up ideas on your competitors list building secrets by joining their mailing lists under another identity - why re-invent the wheel if they have list building ideas you can use? The most important of list building secrets is that it is not the size of your list that is important, it is how you use it to build up your business that matters.