MSN has a much smaller search market share than it is of Google. However, the traffic quality of MSN should not be underestimated. Many SEO experts argued that MSN in general has higher conversion rate than Google. So as long as you have already optimized your website for Google, it cannot hurt to SEO your site for MSN as well.
Factor #1: Title Tags
Title tags are one of the most prominent code elements that webmasters should rely on when optimizing their sites for SERP on MSN. The standard of the title tag is to use about 80 characters.
Factor #2: Content
MSN wants to ensure that the users are offered the opportunities to easily find the content they want. A well-organized site with many web pages will rank better than poorly-organized content with only a few web pages.
Another key regarding content is to always create fresh content on your site. Webmasters optimizing for MSN are encouraged to constantly create new contents, as MSN does not use Sandbox or age filter on newly created sites. For the same reason, your MSN optimization techniques can easily be overthrown by those newly arrived sites that have the freshest content.
Factor #3: Keyword Density
MSN can quickly identify web pages that have to a rapid increase in keyword density within a short period of time. According to MSN, this type of action is defined as spamming and the offended websites are quickly being penalized.
Factor #4: Link Popularity
MSN does quickly give credit to new inbound links, as it does not have an aging period for the links acquired. It means usually in a much shorter period of time after establishing the links (comparing to Google), you should see ranking improvement due to the effect of the backlinks.
As far as the backlinks are legitimate, the more the better. Having high quality inbound links pointing to your site is always great. Certainly, relevancy is also a factor in link popularity. The relevance of the links to the optimization website is a great plus.
A good practice is to build high quality links to your website and your content. This will ultimately allow your web pages rank well not only on MSN, but on other major search engines.
Factor #5: Sitemap