1.Ignoring the Keywords:
This is one of the most common mistakes most people do when building a website/blog. What this will do is..to end you up creating a site with keywords that nobody searches on the web or you could create a site with most searched (competitive) keywords on the net which will make you wait forever to see any results.
So, you should do a quick research on the keywords that most people search...You can use the Google Adwords tool to check whether the keywords you choose have a good amount of searches per month.Also, type your keywords in google search and make sure that you feel confident reaching the first page.
2. Spamming the Keywords:
Some people create web pages just repeating the keywords hundreds of times or blending the font text color with the background color (hidden text)..but, this won't help you increase your search engine rankings.
Also, remember that you are making the content for your visitors, not just for the search engines. Don't repeat the keyword too many times in the article...it doesn't sound right when you read it back.
4. Bad Linking:
You have to be careful on the outgoing links. Linking to irrelevant sites (bad neighborhoods),and sites that have been penalized by Google can seriously effect your own rankings. Also, make sure that you don't have too many outgoing links on a single page. According to Google Webmaster tools, you shouldn't have more than 100 outgoing links on a single web page.
5.Forgetting about Off site optimization:
Lot of people optimize their site well, but they forget about off site optimization...ie, building good and quality backlinks from relevant sites. It is a good way to increase your search engine rankings quickly... Get links from strong sites with your main keywords in the anchor text. Build backlinks by submitting articles, participating in related forums,etc.