You can sell a lot more books if you work with other authors than if you try to do everything all by yourself. — John Kremer
Here are a few tips on how to work with other authors:
Interview them. Have them interview you.
Blog about their book. Have them blog about your book.
Do joint promotions with a group of authors of similar books.
When you are interviewed by someone, ask them to interview your friend as well.
And here are a few other ideas: 51 Ways to Help a Book Author You Love