If you promote your blog in theme related social media websites with good PR then you will get good result on your blog.
It's some time hard to manage the density of long tail keywords especially when your targeted short keywords also reside in long tails.
I think keyword density should be 5 to 7 % but dont repeat keyword tomuch. you can also use synonymous of that keyword
keyword density in a article should be 3 to 5% no more than it and you should read guidelines before submitting the articles.
I totally disagree with keyword density, writing a few keywords around 4 to 5 times in 500 words for me is enough, sometimes you are destroying the article by inserting too many keywords as it doesn't look natural.. a good article has no tricks, readable, natural and also informational or if it is not that way.. it should be entertaining.