We've all heard the phrase "You scratch my back, I'll scratch yours" and it holds true in book promotion. By supporting other authors you support yourself because they will support you. That way both of you are better off and will sell more books. I'll give examples in writing, blogosphere, and twitter.
First of all, writing. That's where it all starts and who better to be a beta reader than another author? I do not consider other fantasy writers to competition. I see them as potential allies; we understand each other's writing because we're in the same genre and so we are also each other's target audience. You've seen that 'People who bought from this guy bought this from this other guy' on amazon, right? That's what this is all about. For instance, I'm reading Dan Wright's "Legacy of the Dragokin" for a review (That review is here) and he's reading my "A Mage's Power" for a review too. We both read fantasy and we both get a new review. Where's the downside?
Second, that other form of writing, the blogosphere. Share each other's posts and each of you will each reach more people. Go on facebook and twitter and linkedin or whatever platform suits your fancy and talk up your fellow writers. Not only does this help the other guy (who is then inclined to share your stuff) but you have more stuff to share and thus more stuff to catch attention. Then there's blog tours and hops; they are powered by sharing. For instance, I joined a blog tour for Dan Wright's "Trapped on Draconica" novel and it remains one of my most popular posts. Certainly it was the one most shared on Google +. It brought my blog greater exposure and so my own posts can reach more people.
Third is Twitter and this is where I have the most experience. I follow other authors and they follow back and so we both can reach more people by retweeting each other's stuff. It only takes five seconds to check their page for the latest tweets and only one second to click the 'retweet' button and send it to a plethora more people. When I joined Book Tweeting Service as a volunteer book reviewer I regularly tweeted about my thoughts on a book as a read it. My follower count skyrocketed. If you keep up with the followbacks (reciprocity!) then the list will continue to climb.
Did you notice the links to Dan Wright's amazon page and to Book Tweeting Serivce's "like a free review' page? Reciprocity!