I know that the more keywords used over and over throughout your content help your site being found in a search engine, but when I write, should I aim to constantly put a keyword in to up my ranking or focus on the writing regardless of the keywords used?
Yeah, I try to at least. But the tough part is when Adsense comes into play, sometimes the ads are different from what is presented. I'll make sure to fit in those keywords without sounding irregular. Thanks.
You can also make a doorway with a huge number of keywords for you. It is actually quite illegal, but sometimes works great! Be really carefull with using it, 'cause you might get bunned (but not necessarily). Good luck!
Focus on writing content for HUMANs. Don't even think about the SE and keyword density other than your title tags and anchor text linking to your site.
After you have finished writing, read it back to yourself, ask yourself if you think it reads ok, does it sound normal, would a human reader find it ok, or have you written it with search engines in mind too much, try and get a balance so that you of course drop your keywords in but at the same time its appealing to a human who is reading your webpage.