It is no secret that the title meta-tag is one of the most important factors when it comes to on-page search engine optimization. Search engines including Google, Yahoo and MSN give quite a bit of importance to the title and description metatags in their algorithms. What many people, however, do not know that the same title tag could actually hurt your rankings, if you try to change it abruptly with a hope to improve your search engine rankings.
If you are currently ranking well with your page title, you should not risk with changing it to improve your rankings. You may often see your current Google rankings go down with such a change. I myself have experienced negative results when I tried to change the title tag for a site which was already ranking well. In fact, Google slammed the site with a -950 penalty.
Your page title is the main entry gate of search engines. Changing the title meta-tag could easily make your Google rankings fluctuate. If you change your title, Google and other search engines will take some time to reflect rankings for new titles. Especially, it would definitely look suspicious in the eyes of Google and other search engines if you have made the changes in the main title and description tags after a long time.
In short, changing the main title of your site, although it’s already ranking well is not a good idea. Instead of changing the page title to try to have improvements on certain keywords, it is better to work on adding content about those keyword phrases on your page, and add links with appropriate anchor text from sites with relevant content. Google always penalized over-optimization. Now it seems as if it frowns normal search engine optimization too. So the best policy is to optimize the website at the time of creation – Do it once, and do it right !

Meta tags introduce your site or blog to search engines. Meta tags communicate with the search engines and give more details about your site. These tags are vital especially if you are running a website or blog to make sure that search engines can easily find your site and index the pages more accurately.

Adding meta tags is very important in order to get more traffic from search engines like Google, Yahoo and others. Adding meta search tags are not the only element for SEO (Search Engine Optimization), yet they still play an important role.

You can find below a detailed description on how to add Meta Tags to your blogger (blogspot) blogs.

Step-1
Log into your Blogger account with your username and password. Once you are in your blog's dashboard click the 'layout' button (see the screenshot below)


Step-2
Look under the Layout tab and click on the 'Edit HTML' tab.


Step-3
Make a backup of your template if you haven't already. This is always a wise idea in order to let you easily repair any accidental mistakes that might happen while you edit the HTML code of your blog.


Step-4
Here is the code you have to add in your template.

Step-5
Find the below code (in the top section of your template) and add the above code just after this code.


Step-6
See the below screen shot, after you add the Meta Tag code.


Note:
Fill out and replace what is in the red as shown below.

DESCRIPTION HERE: write your blog description
KEYWORDS HERE: write the relevant keywords to your blog
AUTHOR NAME HERE: write the author's name (your name)


It is important to note that not to fill the description section with keywords. “Do not do this”. This may result in Google and other search engines banning your site.

Now click preview and view your blog. If everything looks correct and there are no errors, save your changes.

Meta description tags are very important to optimize your blog in search results. With new Blogger interface we can add a meta description tag to Blogger blog without editing template's HTML. But you can not add meta keywords to your blog using Blogger Settings.

Meta Descriptions, which are HTML attributes that provide concise explanations of the contents of web pages, are commonly used by search engines on search result pages to display preview snippets for a given page. (from http://www.seomoz.org)

Go to Settings -> Search preferences.

Click on "Edit" link of "Meta tags" section.
Clicks on a "Yes" radio-button to enable search description.
Enter a meaningful meta description into textarea and save changes.

Now you have add a meta description for your blogger blog.
Note:
But if you prefer to add meta tags(meta description, meta keywords, ...) directly to your template(using Edit HTML), read this article:
http://www.bloggertipandtrick.net/2009/03/how-add-meta-tags-to-bloggerblogspot.html