Obviously, a site needs to have great content filled with SEO keywords and landing pages. But what are the best ways to increase search engine ranking by using other sites/blogs? I have looked into various gigs on Fiverr and also the profiles of several freelancers on oDesk that offer services related to SEO and increasing search ranking via backlinks. But it has been hard to figure out exactly what they do, and I feel uncomfortable asking total strangers earning poverty wages to handle this. I own about 50 different domains that I have no utility other than to help me promote other "real" sites. What are the best ways to use these sites? For example, is it helpful or harmful to setup blogs on these sites with articles that link back to the main sites that I want to promote? Since I own these domains, I would love to figure out how to best utilize them in a way that increases the organic ranking of my main sites.
I would focus on your main site for right now. Make sure all of your citations(NAP - Name, address and phone number) are correct and if you don't have any add them! (Yelp, etc).
Use Google My Business, this will help get your business set up on Google (Not the serps, but Google Plus, etc)
I would start putting out quality content related to your niche and start reaching out to bloggers in your niche with blogs that have high domain and page authority.
I'm guessing the other domains you have probably don't have any domain or page authority, so I don't see those backlinks helping you at all.
Word of advice. You may find gold on those sites, but you're more likely to hurt your site when Google throws another update out and you find they used some black hat methods!