Make it easy for the potential subscriber to use his preference. Purchase ezine ads in carefully chosen publications. Again, make these your well tested ads seeking new subscribers. Keep using the ads that work and stop using those that don't. Swap editorial recommendations with other well written ezines.
Personal recommendations get much better results than just plain ads.
Since you are capitalizing on your readers familiarity and trust in you, don't recommend ezines that offer little real value. The more chances a site visitor has to subscribe the more likely he is to subscribe. If one of the primary purposes of your website is to get new list members, place your subscription opportunity very prominently on your homepage. Use a popup subscribe form to suggest to exiting site traffic that they subscribe. Use caution though, making sure that it does not annoy your visitors.