1. Arrange a sweepstakes with an attractive grand prize, for example one of the more expensive products sold on your site. Require everyone who wants to enter the drawing to give out his E-mail address and agree to receive occasional E-mails from you. To get the best possible results, it is advisable to declare that the winner will be contacted via E-mail and needs to claim the prize within 7 days in order to receive it. By doing so, you'll eliminate the problem of people giving out bogus E-mail addresses in order to avoid receiving advertisements.
2. Open a discussion forum on your site. By only allowing registered users to post, you can collect E-mail addresses and deter pranksters from writing abusive messages at the same time. However, if you decide to use this method to build your list, be very careful. Sending advertisements too frequently to the regulars of your forum may cause them to move elsewhere and thus reduce the traffic to your site.
3. Start publishing a newsletter. Although it requires quite a bit of work, an interesting newsletter quickly gathers subscribers and increases the size of your E-mail database. An additional advantage of having a newsletter versus a simple E-mail announcement service is that you'll be able to submit to the multiple newsletter directories on the Internet.
4. Continue to allow everyone to access most of the content on your site, but also add a "Members only" area that contains articles or other information that is valuable to your visitors. Give out free memberships, but require members to register, reveal their E-mail addresses and allow you to send them an E-mail advertisement every now and then.
5. Whenever you ask your visitors to give out their E-mail address, remember to offer them the possibility to join your opt-in mailing list at the same time. Do you have a form on your site that allows the visitors to contact the webmaster or the sales department? Perhaps you have a Tell-A-Friend script that asks for the sender's E-mail address and name? Both are excellent opportunities to suggest subscribing to your E-mail announcement list or newsletter to your visitors. If you want to gather a sizeable list of E-mail addresses from your visitors, you shouldn't be afraid of trying hard to get them. Just keep in mind that there is a fine line between being persuasive and being annoying.