Post Panda Penguin updates last year things have changed . So be extra careful while doing on-page optimization
First concern is the content . Its must be Original , Unique and informative. Don't spin it using software and use them . It might harm you.
Avoid over optimization , Don't repeat or stuff keywords .
Must not have broken links within website
Other things like Black Hat SEO techniques like
Use of invisible text
Keyword stuffing in meta tags as well as main content
Mirror sites – duplicate sites but different domains
Links from link farmsand questionable sites
Gateway or doorway pages
can be searched within forum.