Do you mean that you want to use the Preview function (“Preview” button in the Publish box) when you have a draft you have yet to publish? If you click “Save Draft” you can either use the “preview button in the Publish box or, if you look above the draft post on a yellow background you will see a Preview post link that looks like this which you can click:
Post draft updated. Preview post
Also if you click the title of your blog in the gray navigation bar at the very top you will be able to view the front of your blog.