Note: this may not be working anymore with latest WordPress and Google Webmasters. Considering this as a historical reference.
Google Webmasters gives two methods for verification: Meta tag and HTML file. The meta tag method can not be used for ones that do not have the permission to upload plugins. And usually theses users such as me on ericzqma.blogs.mu do not have the permission to upload files to the root directory either. But we can use some tricks. The answer is: using WordPress’s Pages.
First, choose the HMTL file method and get the verification HTML file name from Google Webmasters. For example the name is google11e5844324b7354e.html.
Second, add a new page in WordPress Blog named as the verification HMTL file name.
Third, click the verify button in Google Webmasters. It succeed!
Finally, delete the page created or make it private. Then this pages disappear from the pages tab on the blog.
Google Webmasters just verify one time if we do not manually ask for it to reverify.
Then, enjoy it!