Hi,
I would first start off by collecting a list of the blogs/forums or sites in your niche (you can either do this yourself or hire a VA i.e. Virtual Assistant). Then I would visit them (or you can again get your VA to do this task as well) their blogs and see anywhere either in the contact section or if there is any "Write for us" type of link. Based on that, I would contact and try to make some relations with Webmaster of those blogs. Even if there is no "Write for us" sort of link or mentioning of the same, there is still a way - link bait. Talking about it more below.
One another way I recently discovered from a case study by a blogger was also really great and can definitely score some nice and authority links that will stick. What he did was as soon as he added sufficient articles and was decent looking, he set for the hunt to find similar sites related to his niche which maybe providing good information or resource regarding that.
Then he made a resource page on his site and added the good sites which actually provided great value to the readers of the same. And then what he did was just a simple email (using the contact form) to webmasters of those sites like "Hey, I have added a link to your this site or article on the resource page of my blog as I found it really helpful." and in return, he scored some big opportunities. Some webmasters replied back and approached him to do a guest post (notice how things worked even without mentioning the intention to guest post) or either linked back to his site. This is link baiting and you can definitely get some good links from sites related to your niche and even better - the ones which will stick!
However, one thing to remember is to develop or start a relationship (if you are serious about your niche site and see it as future potential income source or want to grow it) and when you get an opportunity to guest post - make sure it is a real quality post and not some 5-15 minutes gibberish write up. Provide an useful content and the returns would be sweet.
Hope that helps.