For me what has worked for MSN is like Sunny Side mentioned, paying close attention to the META tags and also updating your site often. MSN likes new fresh content especially if you are in a niche where the content of your competitor's websites aren't update very often.
Actually, all of the above is good advice, however MSN LOVES clean code. Make sure you validate your xhtml/css and you will do very well with MSN. Basic optimization skills paired with valid w3c code equals high MSN rankings.