The technique is same like you can submit your homepage in different directory sites, blog commenting, article submission, social bookmarking submission etc.
Content Marketing is the best technique after Google Penguin update and avoid Over optimization of site.
Directories, Article, Bookmarking, Blog Post, Press Release submission in relevant categories. Content should be unique when you post and submit any content.
All link building techniques are come under off-page optimization .. This is very important in seo point of view..
you must not have posted your keyword on a relevant site. your keywords MUST be relevant to your website.
Could be any number of reasons unfortunately. Possibly competitors are lapping you in content and link quality as well. Continue to work hard and find relevant links. Quality of quantity.
Maybe links going in to you site have de-valued, or possibly competitor's have gained more quality links. Also possible that their content is performing better.
There are also a number of things you can do "off your web page" to improve your search engine page rank and increase targeted visitor traffic to your web site. This is the area known as off-page optimization. Off-page factors should be carefully considered and implemented to achieve a higher search engine ranking. These factors are primarily tied to the site's linking (inbound and outbound) techniques. This is commonly known as link popularity.
Don't underestimate making flyers and hanging them up at local shops... Probably wont work for adult sites though ;-) I had great return on a flyer for an education site flyer that was handed out to teachers.