Microsoft has tried hard to make themselves more accessible for Webmasters. They are still a little lacking when trying to "self help" - sometimes one source refers you to another, which refers you to a third and the third source refers you back to the first place you looked, none of which help answer your question. However, here is an overview of the 5 ways that you can troubleshoot a problem ranking in MSN or find out more about the MSN/Live Search algorithm.

1. Live Search Webmaster Center Blog - this is maintained by the three people that Live has allocated to this project. It looks to become a useful resource, but it is interesting that the last post (11th January) is dealing with mega search basics, so I'm guessing that they are finding (as we do when we take on a site SEO project) that a lot of sites don't have the basics right. Unfortunately that means that the chances of them covering anything advanced might be a while off, but kudos to them for trying. 
2. Live Search Webmaster Center Forums - You will find the same people here answering queries directly. Can be a good way of getting your site reviewed - follow the general rules of be polite, patient, precise in what the problem is and give your URL. They seem to only be allowed to give out general advice, but there does appear to be a case of "reading between the lines" of the advice they give to find the specific area that is worth extra investigation.
3. Live Search Blog - this is the blog that deals with updates and announcements.  Unfortunately the updates are few and far between, but I guess major changes are infrequent.
4. Webmaster Guidelines - The guidelines have been updated recently, and they are more verbose than before (it was painful to be redirected to something that generalised so much it was useless). They still are lacking in concrete details, but then so are those of the other engines!
5. Direct Feedback Form - The initial response from this is automated, but it is worth persevering to get an answer, even if it just to find out that there is not a ban on your site, so it must be something else.

Live Search isn't well covered in the many SEO blogs out there - a reflection of the fact that referrals to sites are low, meaning it isn't an important engine for many. There is a general lack of testing and reporting of results, which means that people trying to troubleshoot issues with MSN rankings can't find any answer, authoritative or not. This adds to the frustration that can be seen quite clearly in the forums from people whose results are not what they expect.

Updated 14 Feb: You may also be interested in this new blog: If You Don't Rank in MSN/ Live Search How Much Traffic Are You Missing Out On?