Getting JV partners to promote your list gives you access to their list of subscribers. This strategy also helps you to come across as an expert since someone else is recommending you.
You can find JV partners on forums, by contacting them through their website, as well as a variety of other methods. It is often best to contact someone who has a product or website that complements rather than competes with yours. They'll be a lot more likely to promote the product of a person who is not directly competing with them.
Keep in mind that you will often be asked to promote something for them in return, either now or in the future. Make sure you're comfortable with everything they are doing before you agree to promote them in return.
It becomes infinitely easier to attract JVs once you have an affiliate program. Obviously, you'll need your own product in order to do this, but once you do it's easy to get JV partners. Basically what you get from this is traffic to your offers and opt-in box without doing additional work yourself.
As your affiliates drive traffic to your website, you can capture people's addresses before and after the sale. You'll not only make money by having the affiliates promote for you, you'll gain long-term subscribers as well.