In the link below, you can find a good example of how to add a relevance boost for documents that have a specific value on some managed property.
Let's say you have a List, with a content type "Products", and you want products with the managed property ProductStatus == Active to show up higher in the search results.
In FAST you can get this behavior by using the ManagedPropertyBoostCollection as illustrated on MSDN.
What I'm looking for, is a way to achieve this with SharePoint Enterprise Search. Any hints?