There are the two essential elements that are the must haves for every list. One, your list must have value. It means that whatever you offer, it must be valuable to the subscribers. Two, it must be free. This means that besides being useful, your offerings must also be free for the subscribers. Do not try to sell something through such lists. These attempts are not likely to be taken in the right spirit. No matter whichever list building technique you are using, make sure that the list has both these elements.