Buying incoming links:
Although this is a quick way to generate backlinks, buying links from high ranking google websites or “link farms” is serious looked down upon by Google. Google is more intelligent than most people think and knows when a link is artificial or has been brought on  a high ranking site for the sole purpose of increasing backlinks.

Search bots know when a comment is spam or is not comprehensible by humans. This includes spamming comments on blogs or writing crazy blog posts through your content management system with random sentences and keywords in linking back to your site, search engines can pick up on this and they will penalise your search engine rankings.

Over using keywords:
Adding more keywords does not improve your search rankings and can also do your rankings more harm than good. You keyword should be used no more than 3% in your entire page, so use it once or twice for every 100 words you write.

Unrelated Linking:
Your backlinks should all be from relevant websites, so if you run a website that Is related to formula 1 for example and have a back link from a palm reading website, they have no relation and the back link will not count for much.