The default preview in MarsEdit probably doesn’t look much like your blog, but you can improve that by editing your preview template.
While editing, use the Insert Placeholder pulldown menu at the bottom left of the template editor to insert different elements into your template. These will be replaced with the actual values from your blog posts when previewing in MarsEdit.
Note To get your preview looking just like your blog, start by copying the HTML source from the web browser while viewing a post on your blog. Paste the HTML into the template editor. Select all of the text from the post, and delete it. Use the Insert Placeholder popup menu to enter the Body placeholder where the post content used to be. You're off to a great start!
For a detailed description of this process, see the Red Sweater Blog post Perfect Preview With MarsEdit.