Don't use the same Title Tag throughout your site. Try using a unique Title tag for each web page and use your most important keywords in the Title tags that hold theme relevance to that page. Don't use any other META tags such as author, date, etc. unless you absolutely need it. This will just increase your page size and will not do any good to your site. Most major engines cannot read frames so I will advice you that you don't use it but If you must use frames, include important body text within a <no frames> tag. Avoid using completely Flash designs because a majority of major engines can't index flash sites and Flash sites take some time to open fully so it my be frustrating for visitors who don't use a broadband to wait hours for the site to open. Avoid using JavaScripts or JavaScript links. Spiders cannot crawl links in Java Script so its better that you don't use JavaScripts but if you must use it then make sure that you use a text link at the bottom of each page. Keyword stuffing: Keyword stuffing refers to the extremely repetition of a keyword when it is not even needed. Many people stuff keywords in their pages in order to increase their keyword density and thus get a high ranking. But be careful as I said its very very risky please don't try this it may get you banned. Invisible or Tiny text: Many webmasters use tiny text and invisible text in order to rank high by showing lots of text to search engines but at the same time making it invisible for the humans this also viewed as spam. Never submit more than the allowed number of pages per engine per day or week. Each search engine has a limit and exceeding their limits can be seen as spamming the search engines. Do not participate in link farms or link exchange programs. Search Engines have identified them as spam and they don't want you to use it. Don't use "&id=" parameter if you want maximal Googlebot crawlage (many sites use "&id=" with session ID's that Googlebot usually avoids URL's with that parameter). Don't stuff your page with keywords and no content. The search engines will penalize you for this and view your page as spam. Do not put redirects or META refresh tags on your page for any reason. They will get your pages banned from the search engines. A common trick used to be to create a page 100% text and highly search engine friendly but of 0% user value, then have a redirect to the real page. This doesn't work anymore - avoid it. Always remember to use navigation menu at the right side of each page of your site, why? because search engines count the first few phrases appearing on the page, this way you would provide the search engines the content first. Make your site is search engine friendly. Its no use having the best site in the world which no one can find. And for every one's sake don't mess around with hidden layers to get a high listing on the engines. It is JUST NOT worth the risk! Don't link to a bad neighbor, because linking to a bad neighborhood can get your site banned from the search engines as soon as his site gets banned by the search engine the search engine may ban you also.
Yes you also need to avoid duplicate content, but I am not very sure what harm it can do to you. Akash Kumar
Good tip but instead of letting the search engines dictate your design, you can use CSS to display navigation at the top of the page, on the left or wherever else you want it while keeping the navigation code after your main content code.
Expanding on what the guy above me said, put your navigation AFTER your content in the document, but use CSS to display it on the left / top, NOT the right, because your website is more 'usable' that way.
Following are some tips you need on the things you need to avoid while optimizng your wesbite: Don't use the same Title Tag throughout your site. Try using a unique Title tag for each web page and use your most important keywords in the Title tags that hold theme relevance to that page. Don't use any other META tags such as author, date, etc. unless you absolutely need it. This will just increase your page size and will not do any good to your site. Most major engines cannot read frames so I will advice you that you don't use it but If you must use frames, include important body text within a <no frames> tag. Avoid using completely Flash designs because a majority of major engines can't index flash sites and Flash sites take some time to open fully so it my be frustrating for visitors who don't use a broadband to wait hours for the site to open. Avoid using JavaScripts or JavaScript links. Spiders cannot crawl links in Java Script so its better that you don't use JavaScripts but if you must use it then make sure that you use a text link at the bottom of each page. Keyword stuffing: Keyword stuffing refers to the extremely repetition of a keyword when it is not even needed. Many people stuff keywords in their pages in order to increase their keyword density and thus get a high ranking. But be careful as I said its very very risky please don't try this it may get you banned. Invisible or Tiny text: Many webmasters use tiny text and invisible text in order to rank high by showing lots of text to search engines but at the same time making it invisible for the humans this also viewed as spam. Never submit more than the allowed number of pages per engine per day or week. Each search engine has a limit and exceeding their limits can be seen as spamming the search engines. Do not participate in link farms or link exchange programs. Search Engines have identified them as spam and they don't want you to use it. Don't use "&id=" parameter if you want maximal Googlebot crawlage (many sites use "&id=" with session ID's that Googlebot usually avoids URL's with that parameter). Don't stuff your page with keywords and no content. The search engines will penalize you for this and view your page as spam. Do not put redirects or META refresh tags on your page for any reason. They will get your pages banned from the search engines. A common trick used to be to create a page 100% text and highly search engine friendly but of 0% user value, then have a redirect to the real page. This doesn't work anymore - avoid it. Always remember to use navigation menu at the right side of each page of your site, why? because search engines count the first few phrases appearing on the page, this way you would provide the search engines the content first. Make your site is search engine friendly. Its no use having the best site in the world which no one can find. And for every one's sake don't mess around with hidden layers to get a high listing on the engines. It is JUST NOT worth the risk! Don't link to a bad neighbor, because linking to a bad neighborhood can get your site banned from the search engines as soon as his site gets banned by the search engine the search engine may ban you also. Click here to know more on bad neighborhood. Akash Kumar
dont do direct link exchange or huge backlinks in form of dir submissions or news feeds , slow and stedy backlinks are good
The harm happens when the duplicate content is read by a search engine first on another site and then read again on your site. Your site's page(and sometimes entire site) is then filtered from search results. The larger and higher PR sites are usually crawled first and more often so this hurts the small (lower PR) websites more as they are crawled less often. Good luck.