You should look into both On-Page and Off-Page optimization.
1. Know where your back-links are coming from and what keywords are used in the anchor text of these links.
2. You need to monitor your rankings for your main target keywords.
On-Page Optimization:
3. If you want to delete content on your site, make sure you have a 301 redirect to
relevant content on your site.
4. Do not stuff keyword in your blog titles.
5. Make sure each post has its own title tag and description tag.
6. Fix your HTML with Google Webmaster Tools. Use it to find and rectify errors, missing pages, and duplicate title tags.
7. Use different anchor text for internal links. Include targeted keyword, brand name, and generic terms like "click here" or "find out more here".
8. Make sure your site has a Site Map. robots.txt file, privacy policy and contact page.
9. Internal links between pages, to direct your readers to other related content.
10. Improve the downloading speed of your site.
11. Make your 404 page has a link to a relevant page, and not just to your home page.
12. Optimize your images or pictures. Do not just stuff keyword in your alt tags. It should be a short description of the image.
13. Provide social media widgets for your visitors to share your content with their friends and followers via Facebook, Twitter and Google +.
14. Use Google Analytics to check on the bounce rate’. Bounce rate is the percentage of your visitors who enter your site and then leave without viewing any other pages.
Off-Page Optimization:
15. Re-look into links on your own sites. Change the anchor text of these links accordingly, as mentioned in #7.
16. You also need to update links with different anchor text on other people’s sites, such as your signature links in forums and guest posts or comments that you’ve posted on other blogs.
17. Check if a site has been de-indexed by Google, by typing your site URL into Google and click ‘Search’. If it says: "site:(http://name-of-your-site) - did not match any documents, probably your site has been de-indexed.
18. Remove backlinks from low quality sites.
19. If you have been using the same targeted keyword in high quality sites (e.g article networks), then you should balance them with brand keywords and generic terms as anchor text in these quality sites.
20. Request those low quality or spammy sites that are linking to your sites to be removed. Use Whols to locate these spammy sites. Or you may pay people to clean up this mess. For more details check with: Removeem.com.
21. Diversify your back-link building, that include blogs, press releases, forums, Q&A sites, YouTube, and social networks like Facebook and Twitter.
21. Be selective with anchor text. Use targeted keyword anchor text from higher PageRank sites, while use brand and generic anchor text on those less than PageRank 1 sites.
22. If your site has been penalized, submit a “reconsideration request” to Google to have them manually re-evaluate your site.
Let them know in details on what you have clean up or retify.Explain to them reasons why your improved site will improve Google’s results and provide a better user experience for their users.
Submit your reconsideration request to Google at: https://www.google.com/webmasters/tools/reconsideration
[source:www.marketsamurai.com]