I have tried several methods and according to my experiences this is the one that worked perfectly.
Visit the Template option in Edit HTML section and select the Expand Widget Templates option. Now search </body> in the template (with ctrl F) and add the following code just above it
<script type="text/javascript" src="http://apis.google.com/js/plusone.js"></script>
Now add the button code to your blog. Search data:post.body in the blog template and add the following code above or below it.
<div style='float:left'><g:plusone expr:href='data:post.url' size='medium'/></div>
There are four types of buttons available. I have used the medium sized button. If you want different types of buttons then you have to replace medium with small or tall in the above code
Now you have added all the code to the template. Its time to check the Google +1 button on your blogger blog. Click on the Save Template option and check your blog.
