I've been through this. It seems really easy from the outside but when you get in, you're like "Why not promote my launch dude?"
The best and never-failing way is to promote them first. By that I mean, promote their launches, stay in touch with them, and stuff. You them that you're an ideal JV partner who he/she can bank on for years. Yes, I said years.
Some JV partners dont even need communication. "A" gets news of a product launch or offer from "B", and the next thing you know is he is making hundreds of sales for him.... This thing happens a lot, vice-versa too!
There are many ways to promote product launches. Review Sites, promoting to your list, media advertising and many others ... Good Luck!
Thanks!
Sim
P.S From my experience, this WILL take time but it will be good for you and your business in the long-term.