Guest posting is a fantastic way to boost traffic. I use this technique on most of my sites - many of which are authority niche sites. To get the best of both worlds, make sure to ask for a reciprocal link of some sorts. Most webmasters will be happy to do this kind of exchange. It helps to make friends in this world...even if you are selling the same things or monetizing through the same methods.
It may seem crazy to some as many people are often afraid of giving up any precious link juice. In reality, Google is going to see this relationship and reward you with more traffic/ranking. Quality connections are important so make sure that the content of the guest post is quality and if you need to make a change, don't be afraid to ask.
I do this with many websites that I use for Amazon affiliate products and Google affiliate networks. I trade links and posts with other affiliate marketers. To ensure that it won't hurt my rankings, I carefully choose which pages to link to and what type of anchor text to use. In a lot of situations, I have been thanked by the other webmasters because I helped provide some knowledge that they were able to apply to their sites. I have even sold websites to others that I met through reciprocal linking!