The safest way would be to keep giving your ezine away for free and just charge for premium content and services. You can still easily grow your opt-in subscriber base. You won't have to just rely on advertising and affiliate program revenue. Then you could still present your main products in front of a large subscriber base.
There are countless ideas for premium content and services you could offer. I've seen ezines offer ad free versions of their e-zine, extra original content, offer extra weekly issues, etc. You could create a subscription based product just for your subscribers like a subscribers only web site. It could contain message boards, chat rooms, issue archives, free stuff.
The most effective way to find out what to charge for would be to survey your existing subscribers. Ask them what kind of information they are having a hard time finding, if they would be interested in paying for extra content or issues, what type of services would they be interested in, etc.
You could charge a monthly or yearly subscription fee. To entice your current e-zine circulation to convert, you could offer a free trial or sample. You could also make money on the trial subscription by charging a small one time fee like $2.95. If they like it they will subscribe, if the don't they're not losing much money. It may take time to find an offer that works, but it will be well worth it.
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1000 Ways To Sell Your E-Information Products just visit: http://www.ldpublishing.com As a bonus, Bob Osgoodby publishes the free weekly "Your Business" Newsletter - visit his web site to subscribe and place a FREE Ad! http://adv-marketing.com/business