If you're product is up to snuff he will be more then likely be happy to endorse your product to his existing customers and subscribers in exchange for a percentage of the sales generated from the endorsement. On the flip side of the above example if you do not own your own product you can offer other business people the chance to do an endorsement mailing for their product in your newsletter. In exchange, you ask for a percentage of the sales generated from the mailing. Another great yet very rarely used joint venture marketing concept applied online is trading links with websites that share your websites target market. When you trade links with other websites you can easily piggy back on other websites' success and bring in even more targeted traffic to your online business.
Joint ventures do take time to put together but they do bring in results.
But hey, don't limit yourself to the above ideas I presented here today.