Beating My Competitors

My wordpress blog is targetting 3 specific phrases/keyword terms in this niche. When i create new content/new posts should i always try and attempt to include these keywords a couple time in each posts/article? and making them blog/italic/underlined, etc or could Google see this as keyword stuffing, if so whats the rule of thumb/overall percentage on the amount of times you should include your main keyword phrases you are targeting?

Finally if i post new content lets say 2 times a week for about 5-6 weeks (400 words on average for each posts) related to my niche, and socially bookmark (major site: digg, stumple, etc via SENUKE) the new posts everytime i publish them on my blog, should i in most cases see a rise in my websites position in the search engine?

Most of the sites im competing with have little content about the niche, and not really targeting the keywords in which i'm trying to rank well with. They also have hardly any PAGE backlinks, but have a shit lot of domain backlinks. And the only reason they are higher up is because of their authority/domain backlinks and domain age.

Try to use different keywords even better would be to use also long tail words

yes these are long tail keywords

long tail keywords plus unique content plus old domain and you have nice position in serps

I try to keep my main keyword usage to about 5%-7% per page on my blogs. Then as stated above add long tail keywords throughout your posts. This has served me well and has helped me to rank high for very competitive keywords.