1) Optimizing your site for the wrong keywords
The first step in any search engine optimization campaign is to choose the keywords for which you should optimize your site. If you initially choose the wrong keywords, all the time and effort that you devote in trying to get your site a high ranking will go down the drain. If you choose keywords which no one search for, or if you choose keywords which won't bring in targeted traffic to your site, what good will the top rankings do?
2) Putting too many keywords in the Meta Keywords tag
I often see sites which have hundreds of keywords listed in the Meta Keywords tag, in the hope that by listing the keywords in the Meta Keywords tag, they will be able to get a high ranking for those keywords. Nothing could be further from the truth. Contrary to popular opinion, the Meta Keywords tag has almost completely lost its importance as far as search engine positioning is concerned. Hence, just by listing keywords in the Meta Keywords tag, you will never be able to get a high ranking. To get a high ranking for those keywords, you need to put the keywords in the actual body content of your site.