You can add an editor-style.css stylesheet in your theme which mimics the look-and-feel of your blog. Here are some tips on how to do that (you’ll need to touch functions.php too):
http://www.deluxeblogtips.com/2010/05/editor-style-wordpress-30.html
The first two styles in that example are the important ones; usually what I do is just copy over and rename my style.css to editor-style.css, then add these at top:
html .mceContentBody {
max-width:640px;
}
* {
font-family: Georgia, "Bitstream Charter", serif;
color: #444;
line-height: 1.5;
}
Edit for the correct font-family and text color / line-height, of course. This will get you most of the way there.
Here’s another way to go about it, that might get you even closer: http://digwp.com/2010/11/actual-wysiwyg/
Best of luck!